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.
I don't know if I would group those together. Windows 98/2k were visually similar, but OSX by that point was looking quite refined.
It had toned down the blue, but still had some visual flare.
95/98/2k had a very utility appearance. Just compare the recycle bins between each to see how much more effort went into OSX look and feel.