I just picked myself up a Dreamcast at retro game store about a week ago but unfortunately they didn't have any of the Dreamcast games I Recognize as being good. So i decided to pick a random game that looked good and picked up slave zero. Big mistake that game is boring and barely runs on the system. So unfortunately it's only really being used as a CD Player in my room right now. I do plan on picking up Rush 2049 when i get paid and maybe setting the thing to go online and nab one of the online games.
You may already know this, but Dreamcast will play burned disks (backups of disks you already own of course) just try to avoid ones that make lots of nasty disk drive noises since those are usually early rips and will damage your console.
If you're looking for the most players, my guess would be PSO. Also keep in mind that you'll probably have to post online or check to see if others are online if you want to meet up with people, otherwise you may find yourself starting at an empty server