My favourite OSX in terms of visual design was the Panther/Tiger era personally. Leopard looked good, but there was something really cheerful and friendly about Tiger. The iPods of those days were also really well thought-out in my opinion.

Definitely a world apart from the utilitarian Windows 98 UI.

And personally I preferred the XP/Vista era, having search was so much better than not.

However, that just shows how far behind Windows was. Spotlight was much better.