Well, my two favorites have to be Garry's Mod and the Homeworld series.
Gmod because I can litterally do pretty much whatever I want. Build contraptions, make comics, have massive battles with NPCs and whatnot, or just goof off with the tools. There's always something new and exciting to do.
But Homeworld holds a special place in my heart. It's been a favorite of mine since I first got it waaay back. The space combat was great, the storyline, what little multiplayer I did, classic. Homeworld: Cataclysm was pretty good too. Homeworld 2... not so much, but it had a lot of good mods for it.
Infact, I used to work on the HW2-PDS mod as a fiction writer and general community member before the guy that ran it went crazy and practically destroyed the community overnight. A little while after that, I came here. But PDS was genuinely fun until the community tore apart.
