Knights of the Old Republic, by far. One of the best games ever IMO(arguably the best).
Rise of Nations was very good, actually pulled me away from Sacrifice for several months.
Jedi Knight III: Jedi Academy- not as good as the second one, and they should have had some warning that the difficulty had been dialed way back from JKII, but still one of the best games this year IMO.
F-Zero GX- Still having a blast with this one.
I'll leave the fifth slot open, there are a few more titles I've been meaning to pick up that may be up to par. A lot of the highly anticipated titles this year were rather large let downs for me(Zelda WW was flat out bad as an example), either that or games that I really had a lot of fun with didn't come out this year(WarCraftIII, Zelda OoT(Cube port), Halo(PC port, I would count this except I've had it on the XBox for a couple of years now). Looking over all the games I've picked up this year, I've been buying a lot of games that came out in years past that I never got around to getting.