Definitely The Walking Dead for me. Didn't get into the show until it was starting its second season, but I sat down and watched the first season, and thought it was awesome. I like the aversion from calling them zombies, for one. Makes it a little different. Plus the plot and characters are all fairly refreshing.
Mad men..eh never really got into it. Watched an episode, said "I might watch this later," watched anime instead. xD
It's only one of the best things on TV right now, next to Breaking Bad, Game of Thrones and Homeland. It's a drama about an advertising agency in the 60's, which is a point of view from which to depict and break down the entire social context of that era. Plus it's just really well written and Jon Hamm as Don Draper is one of the best leading men in current TV.
Also, according to Wikipedia, it's "the first and only basic cable series to win the Emmy Award for Outstanding Drama Series", which it did four times.
So in short, a much better show than The Walking Dead.
Heard of both but I don't watch either. Having to go based on which one I would probably watch if I was forced to choose I would have to go with Once Upon a Time. Shows like the Mentalist are just not that interesting to me. Also Once Upon a Time looks like it could be OK. I watch too much TV already though.
I have seen one episode of The Mentalist, and every episode of Once Upon a Time. So it's seems pretty clear to me which to vote for. The second season has been much better then the first. I vote for Once Upon a Time.