Unreal was a great refreshing game in the FPs genre in its time, unreal tournament was an awesome multyplayer game based on the same engine and universe.
Unreal Tournament 2004 is the sequel to unreal tournament and runs on a better version of the engine.
Unreal Tournament is still played alot on the internet, as got lots of mods and looks good even on old computers.
Whats so good about GOGs version is that it's cheap, DRM-free and awesome for lan parties.