Argh, choices, choices... I'll make a quick list, and perhaps tomorrow (here is 11:50 PM) I'll make a list for every genre, like Alyxx did.
1) Mass Effect saga;
2) Half-Life saga;
3) Max Payne 1 & 2;
4) Bioshock 1 (the 2 was definetly not worth the time spent playing, an horrible "more of the same");
5) Dead Space 1 & 2 (I'm at Chapter 7 of the second, and it's incredible to see how Visceral Games improved a game that was already near perfection. By the way, if I have to choose, I prefer the setting of the Ishimura instead of The Sprawl, reminds me of Event Horizon with Laurence Fishburne ).