Seriously, where the hell is
Bomberman (the first one)?!
Supports tons of different gameplay modes with a single copy of the game, with up to 8 players.
It's the best multiplayer DS game in my opinion. But I also liked
42 All-Time Classics (30 different games, a single copy of the game needed),
played hours of
New Super Mario Brothers' mini-games with my brother and sister (it's fuuun!).
Mario Kart DS isn't all that bad either, but I prefer multi-card play (more tracks, yay!). I'd happily put
Tetris DS on my favourites list, but nobody I know of either a) hates Tetris or b) sucks at Tetris ... Thank god for Nintendo WFC.
Now put Bomberman on the essentials list (Bomberman Land Touch! kinda sucks, because it features less game-modes than the first one; but it does have WFC support). Now!