Best version of OS X for a 2008 iMac?
I am of the opinion that you should keep your O/S up to date unless you have a SPECIFIC reason to stay at an earlier version. E.G. incompatible software that you MUST run or a noted incompatibility with a service that you can't do without (EG some people are still having issues with MS Exchange compatibility in mail.app on Mavericks).

So, your computer has enough RAM (more RAM if possible) and a nice fast SSD so I would go with the Mavericks install... IMHO