I would have to say Baten Kaitos, it got a really interesting story, the character are awesome
(except for Lady Death, she is just weird) great battle system, wonderful music, this game has it all!
Super Monkey ball Deluxe!
Purely for - Unlimited replayablity
The best game to me is Super Smash Bros. Melee. Various Nintendo all-stars beating each other to a bloody pulp. Plus up to 4 people can play at a time it's wonderful. But the down side is that with so much action going on screen at the same time, it can be difficult keeping track of your character.
Skies of Arcadia, even though it's just an enhanced port of a DC game.
PSO...
just putting it our there.
PSO Ep I&II
PSO Ep III
Super Smash Bros Melee
Legend of Zelda: Twilight Princess (My sister would say Wind Waker)
Mario Kart
Unfortunately I've not played Baten Kaitos, Super Monkey Ball Deluxe or Skies of Arcadia.
Twilight princess,
gotcha force,
PSO (obviously),
Blood Omen 2,
master quest (TLOZOOT),
eternal darkness,
burnouts 1 & 2,
Sonic A2B,
and some others