Best single player: Half Life 2. Playing straight through that is like experiencing a film, only you're doing all the action. In the right frame of mind it's fantastically immersive, the story tells you just enough and the feel of the world is great.
Most pure fun shooter: Serious Sam 1. It's just mad, awesome, huge, fast fun and frantic. Love it.
Fondest memories: Halo CE. THE game I played on my original Xbox. Beautiful set pieces, vibrant, colourful worlds and enemies, well-crafted vehicle physics (heh) - and co-op

The memories of blitzing through Silent Cartographer, Assault on the Control Centre, Library (yes, that level is pretty awesome in co-op if you only use human weapons

), and Two Betrayals with my sister are awesome, and the first time you meet the flood in Guilty Spark 343 was a pretty kick-ass moment in a most unsettling level.
Currently most awesome: BF3 by FAR. I've not been able to play much, what with being off on a laptop most of the time, but the few weeks I've had with it have been phenominal. I love the suppression mechanic, I love the bullet drop, I love the sound-scape, the destructible terrain, the classes, the reasonable balance between weapons, the unlock system, the ability to use scores of vehicles, the variety of ways to play and the size of the maps. I haven't seen a game as technically accomplished, nor one that bring up such tense moments and playing cat and mouse with a tank as the building you're in gets chipped away, or slipping behind enemy lines with a squad and flanking their advance with a wall of heavy machinegun fire.
All of the above is, like, just my opinion, man.