Recommendations for the Dreamcast
This list assumes no familiarity with the console or its games
Jet Set Radio
Jet Set Radio (released as Jet Grind Radio in the US) is a stylish 3D rollerskating game about spraying graffiti, fighting enemy gangs, and running from the cops

The game features an eclectic original soundtrack composed by Hideki Naganuma and Richard Jacques. As well as a number of licensed songs from Japanese, British, and American artists.
The beautiful cel-shaded graphics have aged incredibly well, and still look just as good as they did 20 years ago as they do now. With iconic character designs inspired by 2000s street fashion that are objectively fucking awesome.
If you're looking for a classic you can beat in a couple of sittings then JSR is the first place you should look!
Marvel Vs Capcom 2
Marvel Vs Capcom 2 is arguably the greatest fighting game on the Dreamcast, a console that was the home of just about every golden age fighting game.

A vast and incredibly unique roster, featuring a plethora of classic capcom characters, every X-man you could ask for, and a couple entirely new characters with so much charm it's genuinely depressing they've never been seen since.

The game's jazz inspired soundtrack is incredibly iconic, with absolute bangers you will be singing to yourself for years to come. (River Stage being a particular highlight for me, that horn line will never get old.)

While the new MvC collection has (finally) made this version essentially obsolete, the game is still absolutely worth checking out, especially if you have some buddies over for some local versus fun.