I'm a big fan of the Hitman-series myself ("Hitman:Codename 47", "Hitman 2: Silent Assassin", "Hitman: Contracts", and this spring... "Hitman: Blood Money").
For the ones who don't know what it is, it's basically a stealth game with you in the role of "Agent 47" an professional assassin who travels the world and kills all sort of different "marks".
I very much enjoy the stealth aspect, as well as the open ended level design, which gives the levels great replay value (almost every level has at least 3 great strategies you might use, and a lot of variations of the same strategies).
I also love walzing out from a level having gotten passed all the security unnoticed, killed my target in cold blood and noone being the wiser. I might even have manipulated some of the security to unknowingly help me in my task.
Other than that I also enjoy the Halo-games, though preferably with a friend as I find the whole run-and-gun thing kind of boring without someone to join me in the adventure.
"tellement loin de ce monde..."