I recomend people buy Pikmin as one of their first games and so far, everyone I know who has gotten that game has loved it. You must have it if you have a Cube. It is incredibly addicting.
Also, another one that not a lot of people have recomended is Animal Crossing. I know AC 2 is coming up, but one is still a seminal game, and a lot of fun.
Also, if you love Resident Evil, you must have the remakes.
Eternal Darkness and Metroid Prime are must-buys if you have a cube, as is Zelda TWW.