10.6 is good. I have yet to get 10.7, and i probably wont until software drops 10.6. I never used any of the other OSX oss. OS9 was ok when we had it on a G3 500MHzish powermac.
10.6.8 is a solid OS, I didn't care for 10.5 that much. 10.4 I've had mixed results with, but I still prefer 10.4 over 10.5. I'm running 10.7.2 now and I do enjoy it, however I've had more issues with 10.7.2 then 10.6.8.
My favorite was Snow Leopard (10.6). Lion (10.7) is kinda like Snow leopard with a bunch of useless iOS features thrown in. It adds bloat to the software and detracts from OSX's simplicity.
Apple always has precise road maps. They are going to start requiring all apps from the app store to be sandboxed. It has it's pros and cons, but I would say the concept is pretty solid over all. My opinion is never transition early. Wait until the .3 or .4 release of even the .5 for any machine that needs to be in production and used constantly.