Braid is really one of the first games on XBLA that actually felt like it was made for this type of service rather than simple ports and arcade titles. This game helped breathe a fresh new indie scent into the XBLA and helped set the stage for several other great indie games to come aboard.
2. Band of Bugs
I'm probably the only who would ever dare say this but I effing love Band of Bugs. This game almost feels like the spiritual successor to the underappreciated Gladius games and anyone who even remotely loved that series should definately give this a shot.
3. The Maw
Extremely reminiscent of the N64 and PS1 days, The Maw captures the excitement and awe of a fun and light hearted adventure I hadn't played since Banjo Kazooie or Spyro the Dragon.
4. Battlefield 1943
This is really the one game one this list I feel I could play forever and not get bored with it. Its an easy game to learn and cheap to boot.
5. Geometry Wars: Retro Evolved
XBLA's first real hit which helped spark the rest of this phenomenon. Except for its ridiculous achievements this game is a fun time waster no one should dare miss.