What!?! Only 10 
Everyone has it pretty much covered as far as the top ten goes.
I'll list a little more obscure ones or ones that others haven't mentioned.
The Dreamcast had a lot more accurate arcade ports than the N64 & PS1 produced.
Cosmic Smash Crazy futuristic tennis/breakout hybrid
Bust-A-Move 4 One of the best puzzle series
Gauntlet Legends has extra characters that were later released on Dark Legacy)
House of the Dead 2 Lightgun games! Can't go wrong. Plus it has top notch voice acting
Hydro Thunder Much better Arcade port than any other console for that generation
NFL 2K2 Best as football could get at that time
NFL Blitz 2001 Much better Arcade port than any other for that generation
Phantasy Star Online Where online gaming jumpstarted on consoles and much better than PSU
Powerstone Sega's version of a free for all brawler
Star Wars - Episode 1 - Racer Good racer and better than the N64 version
Typing of the Dead Look to House of the Dead only with a keyboard. Its a lot cooler than it sounds
Ultimate Fighting Championship The last good UFC game, the PS2 games were trash. Hopefully this new UFC by THQ can bring the sport back up to par