The OS that I'll probably stick with will probably be Mountain Lion or Mavericks. I've reviewed, loaded, and tested all of them on an external HD. If Snow Leopard had updated browsing, I'd go with it. Here's my rationale for all OS X versions I'm looking at:
Leopard: Too old, compatibility problems with new stuff, and really, Snow Leopard is just a refined Intel-only version of it.
Snow Leopard: I believe this is my favorite. This had support for old PPC apps, and IMHO it was the last version of OS X that adhered to the "It just works" philosophy.
Lion: Lion to me represents when the "Change for the sake of change" brigade started injecting themselves into the OS development at the expense of the "It just works" brigade. Lion was the last release of OS X that could actually run, and run reasonably well, for me anyway, with 2GB of RAM.
Mountain Lion: I see Mountain Lion as sort of an incomplete implementation of Mavericks. The initial releases of this version were quite honestly terrible. When I first started using it there were so many application crashes and lock ups I switched back to Snow Leopard. Many of the bugs now seem fixed, it boots fast, it isn't that much of a resource hog, but it still has a few glitches that i feel I can handle. You need 4GB of RAM for this to work well.
Mavericks: Mavericks is to Mountain Lion as Snow Leopard was to Leopard. I see it as a refinement of Mountain Lion, but unfortunately it's a memory and drive space pig, which I don't like. For the most part almost every app I have will run under it. It's a toss up for me whether to use Mavericks or Mountain Lion. ML just, for some reason, seems tighter.
Yosemite: If I have to move to Yosemite, I can deal with the looks. They really don't bother me that much, but I hardly call the appearance an improvement. That's not what I'd call high praise. With this release, the "Change for the sake of change" brigade has completely displaced the "It just works" brigade, who now operates under the name "It might work." You may need 8GB of RAM for this to work well, but it can probably survive with 4GB of RAM.
As you might guess, I don't need a lot of the features that integrate the iPhone or iPad into the system. I don't care for iCloud because as far as I'm concerned it's just a security risk waiting to happen.
These are all, of course, just opinions and my observations, so don't take it as the final word or a detailed analysis of everything.