I'd say F-Zero GX is one of the best racing games ever. If you're into futuristic racing that is... It's got tons of drivers (40), plenty of courses (20), and special endings for each driver. Plus there's even a story mode. F-Zero GX is pretty easy to pick up and play, but if you're sadistic (like me) you can attempt to finish the game on every difficulty... and I assure you that's not something you're gonna do overnight. I played the game for quite some time and still go back and play through with another racer from time to time. Easily my favorite racing game for this gen's systems.
I'd also give Mario Kart DD a try. It's more fun with two players, again there's a good amount of racers and a decent amount of tracks, difficulty and replay value. Many argue that Mario Kart 64 was better, and I'd have to agree with them for several reasons. First, I think every track (except Toad's turnpike, unless it was the mirror version) was done exceptionally well in Mario Kart 64. The weapons in the 64 version were done better as well, as Double Dash is more about how well you use the weapons than how good of a racer you are. Finally, the battle mode in Double Dash was horrible. The great 3-D environments in Mario Kart 64 are gone, there's no double-deck or block-fort here. However, you can race with a partner on the same team, and that's rather fun.