I think early Aqua (10.0-10.1) may be my personal favourite, although I don't think I can explain as to why. I remember liking the pinstripes (although I think they were controversial at the time). The 10.5-10.6 look (which was a relief after the brushed metal tsunami of 10.3 onwards) is a close second place and I still use a Snow Leopard box for occasional tasks to this day. I may even have my slipstreamed 10.6.8 disc image still kicking around somewhere.