If somebody has a preference for a release of Windows, it can range. Some love XP, some 7, some even might say "Screw the pushiness - I can make it shut the hell up, and 10 was better under the hood". Hell, I think I've seen comments to the effect of "95 was the best ever".
But having a favourite release of Mac OSX means it's Snow Leopard. I've seen comparisons between $current and $current_minus_one, but never in regards to "That was my favourite" - likely because anyone invested enough in Mac OS releases to have a favourite, favours Snow Leopard.
edit: I must have misremembered as I've been corrected below. Leopard was the last PowerPC version.
10.4 Tiger - Classic (on PPC) and x86
10.5 Leopard - no Classic, PPC and x86
10.6 SL - x86 only
There are some early developer previews of 10.6 on PPC that have leaked and are being played with, but 10.5.8 is the last PPC release.
It's a shame because I really liked my white MacBook. It was a nice machine.