Dr. Mario

Easily one of the best multiplayer games ever, up there with Super Smash Bros. Melee, a bunch of shooters, Crash Team Racing, and Contra. It does take a while to learn. but once you understand what's going on and that it's a race against the person you're playing against, the game is a blast. The object of the game is to kill all the viruses in your bottle of pills. Doctor Mario throws pills in that are colored blue, yellow, and/or red. You need to make lines (usually vertically, but horizontally works too) by matching 4 or more of the same color. After a match is made, the 4+ pill halves disappear and the pill halves that are on any side fall down (which could help or hurt you). So yeah, with those parts that fall down, you can make a combo. When you make combos, the pieces fall into the other players' bottle. The game is crazy fun, and everyone who plays it and starts to get the hang of it (especially in multiplayer) loves it.