Vietnamese banh mi in Falls Church is peerless on the East Coast.
Falls Church is in Virginia, encompassing West Falls Church in Fairfax County as well as the independent City of Falls Church (usually referred to as East Falls Church). Visitors overwhelmingly come for the fabulous Vietnamese and Chinese restaurants; Falls Church is home to the largest Vietnamese community on the East Coast!
The nearest airports are Washington Ronald Reagan National Airport and/or Dulles International airport. There are a variety of airlines (Taca, Lan, American, Avianca, Southwest, etc) Falls Church is located on the Virginian outskirts about 3 miles from Washington.
It's definitely best to have a car, mostly because this part of Northern Virginia is not pedestrian friendly at all. It is possible to get just about everywhere by public transport, if you are a car-less Washingtonian, but your trip will take a lot more time.
A whole bunch of major roads are in play in Falls Church, nearly all of which intersect at Seven Corners, where you had better pay attention to the signs if you want to wind up on the right road! The main roads have at least two names a piece, which complicates things: US-50/Arlington Blvd, VA-7/Leesburg Pike/Broad St, and US-29/Lee Hwy/Washington St. Wilson Blvd is the other main road, where you will find Eden Center immediately northeast of Seven Corners on the way up to Arlington. I-66 runs east-west just to the north of the City of Falls Church; I-495, the Capital Beltway, runs along West Falls Church's western border, with the US-50 exit being the most useful.
Avoid driving during morning rush hour (7:30AM-10:30AM) towards D.C., and vice versa in the afternoon (4PM-7PM). Note that on weekdays eastbound I-66 is HOV-2 only 6:30AM-9AM and westbound is HOV-2 only 4PM-6:30PM.
The Metrorail goes out to Falls Church via the Orange Line, with West Falls Church and East Falls Church stops (right at I-66), but neither are actually in Falls Church, nor particularly convenient for anyone without a car! Ballston is often better if you want to connect to a bus, as it is also closer to D.C. Bus users should make a note of Seven Corners Transit Center, where you can transfer between all the most useful bus routes, and which is just across the street from Eden Center.
Buses 1A, 1B, and 1E  all run from the Ballston Metro stop down Wilson Blvd to Eden Center (just before Seven Corners, past Roosevelt), taking roughly 20 minutes.
Buses 4A and 4H  are the only options for US-50/Arlington Blvd, and only depart from one Metrorail stop: far-away Rosslyn. You can pick them up from the Seven Corners Transit Center, though, across the street from Eden Center, where you can also connect up with the aforementioned Wilson Blvd buses.
Bus 28A  is a pretty cool route, more so for getting to Tysons Corner to the north, but it runs the length of VA-7/Leesburg Pike between West Falls Church and King St/Alexandria Metrorail stations, with a stop at the Seven Corners Transit Center.
Eden Center, 6751 Wilson Blvd, ☎ +1 703 204-4600, . Eden Center, with its colorful arch, cultural vibrancy, and frequent Vietnamese cultural events, is just about the only strip mall in Northern Virginia that can rightly be considered a bonafide tourist attraction. The main reason to come is of course the peerless Vietnamese cuisine in its restaurants, but the shopping can be fun too, be it food or tchotchkes (or just useful, if you are Vietnamese and want some DVDs from the home country). There are more than 100 shops and restaurants packed into this small mall.edit
Falls Church has the best Vietnamese food in the D.C. area bar none, and likely the best in the country east of California (the Vietnamese community here is the largest east of the Mississippi). It's dirt cheap too. The most famous dishes are of course bánh mì (pronounced bañ mee, with a palatized "n") and phở (fuh). The former are sandwiches on semi-hard to hard French baguettes, filled with very Vietnamese ingredients like cold cuts, smoked bacon, head cheese, red pork, various deli meats, or even regular old Italian meatballs. Are there better sandwiches in the world? Maybe—some hamburgers or pulled pork sandwiches might be contenders, but this speaks to just how delicious banh mi really are.
Pho is Vietnamese beef soup (or the U.S. invented chicken phở ga), filled with hot beef/oxtail broth/borderline-consommé, just about any part of the cow you like—brisket, tripe, flank, chuck, round, tendon, etc., and a generous helping of vermicelli noodles. On the side are fresh veggies to add to the soup, in particular mint, jalapeños, and sprouts, as well as lime, spicy sriracha sauce, and sweet-sour hoisin sauce.
Bubble tea and smoothies of all stripes are the most popular beverages (although Vietnamese iced coffee with condensed milk and young coconut juice with fresh coconut meat are hard to beat), and can be found in any bakery/deli, as well as the dessert menus of the restaurants. Bubble tea gets its name from the optional tapioca balls (boba) at the bottom of the cup. For an adventure, try one of the red bean or durian smoothies! The bakeries also have a good selection of strange Vietnamese foods and excellent fresh fruit at cheaper prices than you'll find in any area grocery store.
Vietnamese cuisine, with its fusion of French (thank the colonial past) and southeast Asian, is far richer and diverse than the banh mi and pho alone, though, and you'll find it well represented in the less-specialized restaurants.
There is a smaller, but significant Cantonese community in Falls Church as well, and Cantonese food is a favorite with the Vietnamese, especially for a late night dinner. One fun thing to check out are the dish translations on the menus into both English and Vietnamese!
Note that the Cantonese and Vietnamese restaurants/grocers/bakeries are either cash only, or have minimum purchases for credit cards—usually $10.
2941 Restaurant, 2941 Fairview Park Dr, ☎ +1 703 270-1500, . M-F 11:30AM-2PM,5PM-9:30PM, Sa 5PM-10PM. Chef Bertrand Chemel's 2941 is one of the top destination restaurants in Northern Virginia, with a large, beautiful dining room eclipsed only by the beautiful views over the lake. Cuisine is contemporary American, prepared with exclusively local ingredients.$30-65. edit
Ba Le Bakery, 2822 Graham Rd, ☎ +1 703 876-9062. 24 hours daily. If perhaps not the best in the D.C. area for banh mi, Ba Le in Falls Church is right up there in the top echelon, and is open 24/7. The ramshackle seating is available in limited quantity inside and out on the strip mall parking lot patio (where they even seem to have one of those winterized outdoor seating heat lamps?). If it's 3AM, and you find yourself here eating head cheese sandwiches and washing them down with mung bean smoothies, you are clearly doing something right. Incidentally, you'll note that the Ba Le logo has an Eiffel Tower in it; Ba Le is Vietnamese for Paris—a nod to the French colonial past that gave birth to this magnificent sandwich!$3.50-4.50/banh mi. edit
Banh Mi DC, 3103 Graham Rd, ☎ +1 703 205-9300. 8AM-8PM daily. The banh mi here are absolutely in the top echeleon, and the menu has some hard-to-find variants, perhaps most notably the smoked (not fried crispy) bacon banh mi. The bubble tea is fantastic here, as are the many other baked goods. No seating, so plan to eat on your car's trunk.$3.50-4.50/banh mi. edit
Hong Kong Palace, 6387 Leesburg Pike, ☎ +1 703 532-0940, . Su-Th 11AM-10PM, F-Sa 11AM-11AM. Make sure you get the right menu, and order properly here, as the standout dishes are the spicy Szechuan entrees (like the delicious lamb cumin), rather than the Cantonese, or even worse, American dishes. Unless you are willing to drive to Rockville in Maryland (too far), you won't find better Szechuan cuisine around. Dim sum available Sa-Su mornings.$9-22. edit
Miu Kee, 6653 Arlington Blvd, ☎ +1 703 237-8884. Su-Th 11AM-2AM, F-Sa 11AM-3AM. Notable particularly for those late night hours, this is a really solid bet regardless of the time of day for authentic Cantonese.$8-22. edit
Peking Gourmet Inn, 6029 Leesburg Pike, ☎ +1 703 671-8088, . Su-Th 11AM-10:30PM, F-Sa 11AM-11PM. Ever famous for popularity with the Bush family, who never minded the long trek out to this unassuming strip mall (is all great food in Northern Virginia in strip malls?). Peking duck, naturally, is the star attraction, and you'd be crazy to come here without trying it.$10-25. edit
XO Taste, 6124 Arlington Blvd (just north of US-50 up Patrick Henry Dr), ☎ +1 703 536-1630. 11AM-2AM daily. XO also has notable ultra-late hours, even on ultra-slow weeknights, but also is one of the best bets for Chinese in Northern Virginia. While the menu is long, the specialties are (arguably) the Hong Kong style BBQ and noodle soup. Combining the two, tossing roast pork and duck into your noodle soup, is recommended. Crispy whole fish is a great choice as well. It's in the strip mall west of the Target and Safeway, and hard to see from the main road (US-50).$5-20. edit
Eden Center is ground zero for South Vietnamese cuisine (and culture—the constant South Vietnamese freedom fighter festivals are an interesting look back to the past of this immigrant community). It's an aesthetically unappealing strip mall, but the culinary delights are outstanding, and while you may not recognize them, you'll often be waiting in lines with D.C.'s top gourmet shops.
Banh Mi So 1, 6779 Wilson Blvd (on the east end of the building right next to the arch on the street), ☎ +1 703 534-1950, . 10AM-10PM daily. So 1 is probably popular choice number two for best banh mi in Eden Center. Expect much bigger baguettes and a much longer menu than you would find at, say, Nhu Lan below.$3.50/banh mi. edit
Nhu Lan, 6763 Wilson Blvd (inside the mall), ☎ +1 703 532-9009, . 10AM-8PM daily. There is not a consensus regarding the D.C. metro area's top banh mi purveyor, but if there is one, this is it. It's a tiny little bakery with only four tiny tables and a smaller menu than just about anywhere else, but the baguettes are absolutely second to none. Buy four and get one free.$3.50/banh mi. edit
Cha Kim Phung Bakery, 6771 Wilson Blvd, ☎ +1 703 534-4525, . 9AM-8PM daily. This little bakery is as of yet overlooked by the white folks, but the service is probably the friendliest in the mall, and the banh mi are right up there with the best. They also have a more extensive selection of other Vietnamese baked treats than most of the other delis at Eden Center. Lastly, ginger fans should make sure to grab some delicious unfiltered ginger ale out of the fridge.$3/banh mi. edit
Song Que, . While popular for its banh mi, it might be better to try one of the options above first for the taste test. The best reasons to stop in are the excellent (and varied) bubble teas and groceries. Seating is always plentiful, and there's a really funny sign over the toilet.edit
Pho Xe Lua, 6765 Wilson Blvd, ☎ +1 703 533-3130, . M-Su 9AM-9PM. Eden Center's, and by extension the D.C. area's, consensus choice for best pho (for those who have adventured beyond the admittedly excellent Pho 75 in Arlington. Even many West Coasters admit this is the best they've had outside Vietnam. There's not much more that needs be said, aside from cash only, and a bit more expensive than other pho servers (but absolutely worth it).Small bowl: $7, large: $10. edit
The following all have traditional table service and full bar.
Huong Viet, 6785 Wilson Blvd, ☎ +1 703 538-7110, . Su-Th 10AM-10PM, F-Sa 10AM-11PM. Virtually everything on the gargantuan menu is great, although the more adventurous in the delicacies/specials section are the most rewarding. The most famous dish here is the crispy quail wings appetizer. This is also the oldest restaurant in the mall.$10-22. edit
Phung Hoang, 6795 Wilson Blvd (hiding inside Saigon West—the southwesternmost corner of the parking lot), ☎ +1 703 538-6732, . 9AM-8PM daily. The restaurants inside the mall are harder to find, and attract a more exclusively Vietnamese clientèle. The standout dish here is bún bò huế, a richer vermicelli soup than pho, with a reddish, chili-heavy broth, some congealed pig's blood, a strong taste of lemongrass, and some optional fish sauce on the side, in addition to the same fresh veggies associated with pho. Full bar.$7-18. edit
Thanh Van, 6795 Wilson Blvd (inside the mall), ☎ +1 703 639-0901, . 9AM-9PM daily. Vegetarians rejoice, Eden Center has a Vegetarian/Vegan restaurant! There is a full menu, but the biggest draw may be the vegan pho with really convincing imitation meat.$6-18. edit
Viet Taste, 6763 Wilson Blvd (inside the mall), ☎ +1 703 531-0011. 10AM-10PM daily. Another great, traditional full-service restaurant with a long menu to rival Huong Viet's, but an especially fun choice in the evenings, with frequent live music. The hot pots are a specialty.$8-20. edit
Vy Bistro, 6757 Wilson Blvd, ☎ +1 703 538-7575, . 10AM-9PM daily. Sleek, modern, and trendy, this is one of the best of several more upscale restaurants serving modern, more creative Vietnamese cuisine, which is pretty hard to find the U.S.!edit