It depends what you like.. I downloaded the following..
Mega Man 9 (epic win)
Tetris Party (the game is good the players are really good and with items on wifi it has like 6 players at a time)
Dr. Mario Online RX( also pretty good but the players online are really good you can send a free demo to play with ppl that dont have the game using wifi +10 points 4 that)
Contra Rebirth (meh you may wanna skip it it gets boring fast, and they left the debug code out of the US version)
Bubble Bobble Plus (its good but a ton harder then the other games in its genre)
Gradius Rebirth (it wasn't bad but got boring like contra but it has a leaderboard so its in the middle)
Final Fantasy 4 the After Years (it is really expensive with all the DLC but it is in the league with Mega Man 9 its epic)
Oh and truthfully I haven't seen anything on the DSI ware that would make me buy a DSI.