I'm thinking of buying Rayman 2 since it's on GOG but I can't seem to find in the description or the reviews anything about what version this one it's based on, or if it's just a straight port of the PC version of 1999.
As far as I know, the near-remake on the PS2 is a straight upgrade over the original n64 release, but the Dreamcast one is what the developers say is the best port.

It's basically just the original PC port. Also in my opinion, the Dreamcast additions are negligible and I don't think the Level Select on the Dreamcast version is as charming as the original one so the PC port is one of the best options still.