I've been a Mac head since my first B&W Mac in the 80's. So my first windows PC was purchased exclusively to play Unreal Tournament. I was blown away by that game, I remember looking at the reflections and the multiplayer game play with my jaw on the floor. I really haven't played much in the way of games since then...
Until recently...the onslaught of thousands of half decent flash games that are fun and entertaining. No fancy graphics needed. In fact, most of them could have been played on an 8 bit system if properly written. The best one I've seen lately is here: http://ec2-1.playr.co.uk/onslaught/ Screenshot of the action below: