So as the subject asks, what game/s define what the Dreamcast is all about?

I'll have to start off with Quake III Arena. Being the first console First-Person Shooter to have online multiplayer and all, it really boils down to the Split-Screen for me. It's where I spent the majority of my time with the game. Having everyone around the Dreamcast and making all that noise is always sure to create the best memories. Although, let's not give ALL the credit to Quake III Arena as there are many awesome 4 player games on the Dreamcast to have a blast with like Spawn: In the Demon's Hand.

The next one is kind of special, Sonic Adventure 2. It brought many platformer elements and paired it together with action. It's a game that for it's time, really brought new ideas to the table and allowed us to enjoy groundbreaking experiences [such as the truck chase or raising our own Chao]. On top of the main story, you have the multiplayer modes and most importantly, CHAO GARDEN to keep you busy while you take a break from the main story. It's like this game was designed to make the players keep the game loaded on the Dreamcast clocking as many hours as possible.

I would like to mention more but it would take away from what I have already posted.