For me, it's Rome: Total War hands down even before the patch.

I'm lucky in that my interests span several genres. I thought 2004 was a banner year for the PC.

UT2004 and HL2 are excellent. The latest Call of Duty is a blast. Doom3 is fine. Pacific Fighters continues Oleg Maddox's domination of the flight sim genre. Pirates! is captivating and Riddick EfBB is surprisingly good. BfME is a fun RTS.