Skip Sonic Rush unless you're a hardcore Sonic fan. It's a good game, but it has some poor design choices.
If you like challenge, check out Trauma Center: Under the Knife.
If you like puzzlers, Tetris, Meteos, and possibly Konductra are all must-haves.
If you like first-person action RPG dungeon crawls, check out Deep Labyrinth.
If you like quirky, off-the-wall RPGs, check out Contact come October.
If you like puzzle-solving adventure games, check out Trace Memory, Touch Detective (October 10), and Hotel Dusk (Winter).
If you want some more unusual gaming experiences, check out Electroplankton (love it or hate it software, but I love it), Brain Age, or Nintendogs.
If you like strategy, Advance Wars.