Hands down: Georgetown -- Sushi Ko
Dupont -- Sushi Taro
Get reservations for both, esp. if it is a big group of people. Both are excellent and you won't be disappointed (just be prepared to spend some dough).
If you want decent all you can eat sushi, check out Nooshi on 19th Street, just south of Dupont. Between 5 and 8pm they have a sushi happy hour for $20 or $25 (the $25 has a few extra items on there) and they give you miso soup and dessert. It's decent, esp. for the price. And you probably won't need a reservation.