I would say Tiger, except, it is hard to remember what it was like to use it on a day-to-day basis. I really like Spotlight's implementation in Tigerfor me it's gotten worse since then. I liked the results window. And I don't use a lot of the extra features Apple has added since then. Still, as far as a more modern OS, Snow Leopard is wonderful, and the philosophy behind it: just making it cleaner and better was pitch-perfect.
I would love to see where Bertrand Serlet would take Mac OS X at this point (and I purposefully called it Mac OS X, I'd like to think he'd put the Mac back in the name and the Mac back in the Mac).