Best 2+ player game for the DS?

[Scarecrow]

In your opinion, what is hands down the best 2 player game available on the DS that it's actually worth buying a second copy of the game? I have a number of games and a few have 2 player options except, you need another game card... Now I love playing multiplayer games and I wish they utilized the download play more often but they don't.

Anyway, I wanna get a game that's actually worth buying two copies so you can go head to head against someone else? (which reminds me... no co-operative 2 player mode... you gotta be against each other)