My favorites:
Crimson Skies (probably my fav original xbox game - this never saw a sequel which must mean it sold badly, because it looks fantastic today and is a superb game)
GTA III and Vice City
Psychonauts (disclaimer - I am stupid as I haven't come close to finishing this brilliant funny game)
Panzer Dragoon: Orta - maybe best graphics of its generation? Fun on-rail type 3d shooter
Outrun 2006 Coast to Coast
Ninja Gaiden
Beyond Good and Evil
NHL Hitz 2003 (soso single player but fantastic multiplayer)
Baldur's Gate: Dark Alliance
Burnout 3: Takedown
Soul Calibur 2
Fable
Halo 1 and 2
Oddworld: Stranger's Wrath
NFL 2K3 (I think that's the year? Still the best NFL game IMHO - edit: I think it is NFL2K5)
Star Wars KOTOR - but you'll need to invest a lot of time on that one! Buggy in parts but absolutely brilliant and no, I haven't finished that one either!

Quantum Redshift (wipeout type racer, well I liked it...)
I'm forgetting something obvious I'm sure...
I think a game like Forza is pretty awesome but because the series is still ongoing and especially because it's grown so much aesthetically, I personally don't have any desire in revisiting the first game in the series. Actually I feel like that for most (but not all) 3d racers. For some reason, I don't feel that way with Burnout (though I do like Burnout Paradise on the 360, Burnout 3 is still awesome in its own right) and Outrun 2 ... Weird huh?