Mr Driller
Soldier of Fortune
MSR (Metropolis Street Racer)
Sonic Adventure
The fishing games are also great fun if you get yourself a fishing controller.
Do not under any circumstances buy Rainbow Six as it is dire.
You should check out the DC emulation and homebrew scene though - there are loads of great emulators and ports for the DC and you do not need a chipped console to play them.
Dead or Alive 2
Out trigger
Headhunter (one of the most amazing DC games released)
Crazy Taxi 1 + 2
Silver
I had a complete DC collection (not all originals :unsure: ) and I have to say that out of the 300 or so games there were, it's probably easier to name the bad ones than the good ones because there were so many titles worth playing.
Games to avoid:
Fighting Force 2
POD
Nightmare Creatures 2
SPEC OPS II
Aside from the few above, nearly everything listed on Planet Dreamcast is worth a try.
bomberman online (brilliant 1 to multiplayer game)
samba de amigo(even without the marracas)
powerston 1 + 2
typing of the dead
tokyo bus guide
virtua tennis
illbleed ( a bit shit but i liked it)
shenmue
boot disc
Project Justice
Bangai-O (made by treasure)
Mars Matrix
Gunbird 2
Granddia 2
Record of the Lodoss War
Marvel vs. Capcom 1 & 2
Just to add to the other great titles that have already been mentioned in this thread.