I don't think there is really a favourite genre of mine, but judging on the amount of games played, I'd have to say simply "shooter" - be it first person (Serious Sam, Crysis 2), third person (Rune, Jedi Knight II) or side view (Iji, Soldat). Most of these games do not require much thinking, allowing to either plow effortlessly through hundreds of monsters or engage in quick skirmishes with small groups of enemies - a perfect way to just turn your brain off for a moment, forget about everything and get relieved of stress. Generally I'd even review "shooter" to "anything that provides some good action and allows you to kick some ass" - this would include e.g. space sims - Freespace 2 was simply glorious.

Second... RPGs, I think. Exploring the world (yep, I gotta say I'm attracted to sandboxes), developing character (create someone You cannot be in the real world! or someone You could be, if You only had more balls!), making choices, interacting, blah, blah, blah.

Third would be strategies. I've played more real-time than turn-based, but well, both subgenres have good and bad titles. As anything.

And as for X-COM, that game is pretty much immortal. I've downloaded it recently after seeing a fan-art of sorts - and whoa, once again I kinda disappeared from this world for a few days.