The 1998 Unreal game (the one which the famous engine is named after, before the Tournament spinoff games). It was my passion since I played a beta version back in the day, one that isn't floating anywhere online, but yea it was my passion recovering lost content for this or the playstation version that I started to re-work as a mod and various addons.
Unreal Tournament was also originally just an addon for Unreal, there were two betas that leaked back in the day too and the earlier one is still "more Unreal" than that(from the time right after "Unreal: Tournament Edition" plan, though the full expansion pack/botpack phase of it didn't get out.