On my Dreamcast, there were very few games I played as much as Unreal Tournament, Spawn: In the Demon's Hand, and Phantasy Star Online (especially this version). It was an excellent blend of fun and challenge, and had a gameplay loop I could get into. The current MMO overcomplicates with gratuitous monetization and far too many NPCs for shops and crafting. But there's good entertainment value in the simplicity of PSO ver. 2. While it probably wouldn't be multiplayer on GoG, I mostly played single-player anyways. While it'd be retro by today's standards, it definitely holds up.