GoldenEye for the Wii is fantastic. 

Portal 2 is coming out the 19th, Deus Ex Revolution comes out in august, Battlefield 3 comes november, but if you want something that is already out, call of duty has short and predictable story lines. I would get the orange box if I was you.

Half-Life is overrated but you should play it anyway. Overrated doesn't mean bad.

Also, you I don't think you mentioned a platform so I'll suggest anything - like Crysis, for example. The first one was pretty awesome.

Bioshock and Bioshock 2
Halo 3 and Reach
Resistance 2
Singularity
Orange Box
Prey
The Darkness

I came in here to recommend Halo: Reach, Half-Life, Bioshock and Borderlands.

I'm leaving thinking I need to buy me a copy of Singularity.

killzone 3

its single player campaign makes you don't wanna stop, you will get to use lots of different element through out the whole game, and its all really epic, it won't get bored, the more you go the more excited you will get, trust me... ^^

Killzone and F.E.A.R. series

I would advise you to stay away from Call of Duty if you like single player experiences over multiplayer.

I definitely recommend Bioshock. I love messing around with the plasmids, I love Rapture, and I love the fact that most of the game's story is told not via cutscenes, but via audiotapes, that don't take control away from the player.

Definitely BioShock. It's up there with the greatest FPS games of all time for me, and this is coming from someone who doesn't really like the genre too much.

Bioshock

Resistance 1

Borderlands

I recommend these not only because they are fun, but in the cases of Borderlands and Resistance the campaign length is actually pretty damn good.