I've been playing games for years and years (I had an original NES when they were still sold in shops!) That having been said there's not that many games that I would consider favourites, or rather I can't think of that many games ... lets just see how the list goes.

I'm a HUGE fan of the Ratchet and Clank series on the playstation (points to avatar ), the first one I played was Tool of Destruction on the PS3 and got the rest from there.
I've really enjoyed the Mass Effect series so far, impatiently waiting for number three.
Same goes for Gears of War, I've just started playing a co-op game of GoW2 with a buddy in insane difficulty, we've come to the conclusion that it's our Kobyashi Maru test, LOL!
Metal Gear Solid is a game series I keep going back to, despite it's impenetrable story line and FMVs that requite a loading screen half way through.
Portal and Portal 2, completely different from pretty much anything else I've ever played and hysterically funny as well.
I thoroughly enjoyed Psychonauts, again because of the humour.
The Mario series has always been pretty good, though I prefer the older games, Super Mario World and Yoshi's Island both on the SNES I think are my favourites, though I also really liked New Super Mario Bros on the DS
And my favourite game of all time has always been Sim City, particularly the SNES version, though I got on quite well with Sim City 3000 as well. I have it on my iPad as well so I have something to do during my more boring lectures at college, LOL!