I'm going to base this entirely on my own opinion but success to me is when someone is doing incredibly well in their field and living a positive, happy life.
In that case the two definite and obvious contenders are:
Alyson Hannigan (happily married with a family, steady TV career, no negative media coverage)
Tony Head (combination of stage and television work, family, works tirelessly for charity)
Points could also be argued for:
David Boreanaz (Bones has been incredibly successful and in my opinion he has improved 10 fold as an actor, however he hasn't always had favourable media coverage due to cheating on his pregnant wife and apparent egotistical ways)
Danny Strong (his name seeems to be becoming more and more well known due to his writing and directing career)
Let's not talk about poor Nicholas Brendon who has not done any work worth mentioning in a favourable way since his struggle with alcoholism and has spent 10 years being well known as the guy that used to play Xander and is now permanently getting arrest. Not to mention he is now divorced and appears to be single. Poor sod 🙁