What game should I get?

[Icemario]

Out of all of the Mario games you don't own, get Mario & Luigi: Dream Team. It's- dreamy!

Seriously, though. Mario & Luigi: Dream Team has great visuals, an excellent soundtrack, top-notch humor, very good gameplay (basically the same as other Mario & Luigi games, which is great), memorable characters, references to other Mario & Luigi games, fun Dream World areas, an entire island to explore, a great concept that works, and lots to do outside of the main game (including a Hard Mode). The game also brings back several features from past Mario & Luigi games, including special Bros. Moves, Giant Battles, 2D exploration (in the Dream World), and X Bosses. Overall, this is probably the best Mario & Luigi game to date.

I second this.

Thirded, plus it actually has an in-game Jukebox once you beat the game, which they need to do more often.