i'll second nearly all the recommendations made so far. i've played xbox and ps2 quite a bit, but there is something special about Nintendo-made games that just cannot be imitated.
Metroid Prime - now $20... a steal for that price
Mario Sunshine - I like this game more than a lot of people did, but it deserves my personal recommendation.
Smash Bros. Melee - best multiplayer game ever once you get into it. it took me a while, but it's been a blast since.
Super Monkey Ball 2 - Decent single player mode, tons of replay value for multiplayer.
Pikmin - Many original gameplay elements... cheap price right now, but you might want to wait for the sequel coming out soon.
Eternal Darkness - Underrated game.
keep it