FPS: COD4/World at War. Haven't personally tried World at War, but if it's as good as 4 was, they're both worth picking up. Ultimately, choosing between one or the other comes down to which theme you prefer.
Left 4 Dead was EXTREMELY fun when I played the demo. Really had a unique experience.
RTS: C&C Tiberium Wars is good if you don't like economic micromanagement. Tiberium Wars is very very good if you'd rather micromanage the actual battling aspect of winning. Also, the games are very fast paced. If you use proper rush strategies, and your opponent doesn't counter, games can end in 5 minutes.
MMORPG: You have to try World of Warcraft once. I've played Guild Wars & Warhammer, and while I like both, ultimately WoW is still my choice for MMOs. I wouldn't really recommend them though. Playing MMOs like these are like being brainwashed, because you'll end up so obsessed you won't use your mind for anything other than thinking about WoW. If you want a great gaming experience, get to 80 and play some end-game content in WoW. But if you end up doing it, don't say I didn't warn you.

These are all recommendations based on games I've played. Based on reviews, I still really want to play Mass Effect and Fallout 3.
These, in my opinion, are currently the top titles.
Cheers!
Edit: If your goal is to sit for hours and hours, then WoW is the game for you. If that's a non-issue, dive right in. Just research classes and races before you pick one. Also, take advantage of the recruit a friend program. Google it. Simply put you level much much faster!