Infinite Dungeons 3, even if it never got finished. The combat was epic, the style was epic, the dungeons themselves were epic (they eventually got ported to ID2)... I can't name every last cool thing.
My favourite would probably be Hands by Bromeo_Wuggles. It's neat playing hide and seek when you make the map.
I also have a plot I'm working on I really like (I mean, why wouldn't I like my own plot). It's in alpha, but my goal is to have a game that uses nerf dart guns which you can customize in a fancy GUI, and maybe it could play similar to TF2. I don't really know where it's going though.
Death run will always remain one of my personal multiplayer favorites. Coming home from school after a long day and playing with the community & crew has made some of my most enjoyable experiences lol, I think a lot of it is just nostalgia. Another favorite was that one game that N_Enders made, trivia murder party!