I'm contemplating which one I should get. I found some pretty active servers on UT99 and 2004. Not so sure about gold though. Anybody have a personal favourite?

You can't go wrong with any of those. Unreal Gold has one of the best single player campaigns of the '90s. UT99 and UT2004 are first class multiplayer fps that feel quite different. Try the demos and see which you like better.

For single player, Unreal Gold. I never play multi-player online, but still have the other games since they include bots.