XP. If you have any DOS games or even some old Win9x games you still want to play you might want to dual-boot with W98SE (or whatever 9x you have) and have at least one FAT32 data partition.
I've read of people being unable to run classics like Planescape: Torment under XP, and I know XP is as hostile to DOS soundblaster emulation as Win2000 is.