My vote is for OS X 10.6 Snow Leopard. Without a doubt, the best OS X version Apple EVER developed and designed.
This is my everyday running OS X version.
A big thanks goes to the senior OS development group leader, at the time, Bertrand Serlet. I really miss him at Apple.
I have, also, installed OS X Mavericks. Mavericks has some cool features that I like, but there is a lot of the Apple apps that have been simplified to death that have a lot of missing functionality compared to the older Apple app versions (namely iMovie and all of the iWork apps), third party apps I can't afford to ever update that will keep me pretty much forever on OS X Snow Leopard. If there ends up being some sort of major and serious security breach that happens to Snow Leopard, I will just stop connecting to the Internet with my IMac when using Snow Leopard and will have to use Mavericks more and use apps I was able to update/upgrade for free or cheaply and access my email and Internet from OS X Mavericks and/or from my iPad.