For me its tricky, I have phases where Ill love a single game exclusively, but in general...
-STALKER CoP, I haven't even beaten it and let this one game top my list, that is how much I love it
-Battlefield (3 & BC2) favorite Fps by far.
-Demons Souls and Dark Souls, these are equal to me.
-Fallout 3, New Vegas has nice additions, and I may be pretty "sick of" FO3 by now, I still like it.
-Oblivion and Skyrim, good for that "Open world mood", Im effectively tired of Oblivion even though I haven't beaten the main quest, Skyrim is beautiful but like Oblivion, Ive barely touched the main quest line.
Shoutouts- Company of Heroes, my favorite RTS. Red Orchestra 1,second best fps, really hard to do well though. (OHKs for rifles?Take that CoD!)