mine, currently, is Minecraft...
but, not much of a serious gamer, and more just build random stuff and similar, generally in single-player.
in the past, it was Quake 1 and 2.
with the source available, these gave a lot of room mostly for fiddling with stuff...
but, ultimately, the same maps with the same monsters got a bit empty-seeming.
and, they were preferable IMO to Quake 3 Arena, which lacked good single-player gameplay, and either running around empty maps (and shooting walls or similar) or running around being shot at by bots, wasn't particularly compelling (never really got big into multiplayer or deathmatch or similar, mostly because I tended to burn out over the course of a few minutes). though, Q3A largely overshadowed Quake 1 and 2 in terms of popularity.
I actually preferred Quake 1 over Quake 2, mostly as Q1 had more "atmosphere"/"personality" and found the QuakeC development slightly preferable to rebuilding the game DLL. eventually, I ended up with a state of using the Quake1 maps, with a modified Quake2 engine (with a lot of parts of the Quake1 and Quake3 engines glued on). this effort was later dropped, and my own engine was written, mostly to get rid of the GPL.
much of my architecture and technology was strongly influenced by these engines though, like, if not originally for QuakeC and the Q3A JIT, and a chance encounter with a web-development class, my BGBScript language would probably not exist.
well, and my current BTIC video codec and use of AVI video-textures ultimately owes part of its existence to the Id ROQ format (though, there are more subtle reasons I didn't just reuse ROQ). (well, and it doesn't help that I don't have much "original content" to use in them, so mostly it is a mix of random things like putting random videos off of YouTube onto textures, and/or using them mostly for utility things, like animated textures...).
other parts were influenced by Minecraft (such as the use of voxel terrain), and also things like Mozilla Firefox, various information about some Adobe products, ...
yeah...
but, ultimately, "the magic" hasn't really been captured, and no one cares, ...
so, I exist mostly isolated and alone, operating inside a void...
the rare few people who say anything basically just hate on my stuff, and there is little hope of anything worthwhile happening as a result.
and a lot of my custom DCC tools and similar suffer from "awful UI" issues, so have little hope of being relevant to anyone, ...
or such...