Well my personal favourite is Call Of Duty Black Ops. The game has intense graphics which makes it very realistic...
Counter-Strike Source is probably my favorite. But recently I've gotten into the Call of Duty games. I always seem to return to CS:S though.
my deskmate in high-school was crazy about DOTA, took me to a net-cafe where everybody was playing it to introduce me to the wonders of DOTA. I ended up getting the entire party killed. And then i gracefully retired from playing DOTA
I play World of Warcraft a lot so that has to be my favourite to be honest, it's a great game, I much preferred the Wrath of the Lich King expansion though, cataclysm isn't as good imo.