Tim Russ is the only recurring actor i can think of right now that has been in many of the different series as different people.
or...
the same very old person, although I think he is a human on the bridge of the enterprise-B.
apparently there were lots of them, i hadn't noticed. mark lenard (spock's dad) has been different characters, so has marjel barrett. don't forget the nick locarno to tom paris travesty that was, imo, just lazy.