Well, I used to game a lot more than I do at the moment, or have done in the past few years.
But, with that said, then one of my all time favorite games if not THE favorite one that I have played and still play from time to time, has to be Dungeon Keeper, the original one. By far the most fun I have had.
Had my crush on Diablo as well, especially the first one, but Diablo II was also quite good.
When I do make the time for it, then currently, I enjoy some Vintage Story, rather like the whole Lovecraftian theme it has, as well as the survivor and crafting system.
As for the MMORPG genre, then I have to admit that I have been around quite a few, from EverQuest, EverQuest 2, Anarchy Online, Lineage 2, WoW, Istaria, Asherons Call 2. Star Wars Galaxues and SWGCU (did not like the CU though).
Who knows, perhaps one day we will be able to set something up through here for people to possibly connect and play something together.