Virtua Tennis is generally given the nod as the standout sports game on Sega Dreamcast. There’s also Virtua Tennis 2.
Tony Hawk’s Pro Skater and Tony Hawk’s Pro Skater 2 are both classics, and although they appeared on Dreamcast, people tend to associate them more with PS1 and N64.
If you’re talking traditional team sports….
NFL Blitz 2000 and NFL Blitz 2001 are classics, but they’re also associated more with PS1 and N64.
Most people will tell you the Dreamcast’s best sports game is NFL 2K2, followed closely by NFL 2K and NFL 2K1.
Some people are also fond of NBA 2K, NBA 2K1, and NBA 2K2.
Not to be a sourpuss, but I’m not a big fan of any of the 2K games on Dreamcast. Their gameplay seems dated and bland when you play them today. Graphically, they were impressive for the time, though. I’ll give them that.