Shenmues, Seaman, Skies of Arcadia, Code Veronica, Powerstone 1 & 2, D2
And if you want a bizaare difficult game: Illbleed.
There was also a space shooterish mech game that was fun, but I can't remember what it was...
and some hack n slash game that was fun: Sword of the Berserk
I liked Trickstyle. RE code veronica. NFL 2k1, Sonic Adventure, Vanishing point, Crazy Taxi, Capcom vs SNK..or maybe it was Marvel, can't remember. Ecco was ok as well. That's all I can remember for now.
I know Dreamcast has some of the best fighting games, but they've just never been my thing.
Panzer Dragoon Saga
Albert Odyssey
...are not Dreamcast games. They are Saturn games. Wrong Sega console.