2000Pro, firm believer in it. I've used 95, 98SE, ME, & XP too but I love 2000.
Have had a lot less issues with it, never any crashes (unless it was my doing). Never had any security issues so far. I am speaking of just Home use. It's not a resource hog either which is good in my respect as old as my computer is. Plus I've learned that most programs that call for XP will work on 2000. There are exceptions of course.
There are several good Linux OS floating aroun too. I've tried Ubuntu but my computer just don't have enough umph to it. Also ZenWalk, now it ran fine. You can also get live CD versions that run right off the CD so you can check them out before you install.