98 no question. Neat, functional and not too intrusive. I have XP setup in 'Classic' mode so the icons and menu's all look like 98.
I just don't get on with the cartoony, fuzzy app heavy user interfaces like the default in Vista. Don't see the point, all it does is hog extra system resources and doesn't make using or browsing the PC any faster or easier. A good UI should always be about efficiency and utility rather than eye candy.
I'm using the default Vista theme.. I don't really have a problem with it, and I really like the 3D accelerated effects of Aero.. As long as you have a decent system with components from the past couple of years it's not really going to noticeably impact your performance.
I liked 98se vs earlier versions as it has a more smooth feel to some of the animations and some of the icons were a little touched up. XP in classic is also great, but I haven't really touched much of the default Vista stuff.. I do hate the cartoony default XP theme but Vista is a little better. I don't know if different versions have different defaults though, but I'm using x64 Ultimate and all is well. The animations are smooth and the transparent effect is nice too. No real hit to my performance. All I've done is change the start menu look back to the more classic one with small icons, and disabled personal settings, and annoying taskbar shit like hiding icons.