Just getting back in to the game after a long hiatus, and I like both gank-heavy junglers and supports-that-aren't-healbots. I was going to go for Zac but, just looked at Thresh's abilities, found out he can be played in...a good variety of roles. Either champion more viable? Not OP, just...more accessibility in a variety of team comps, both solo and organized.
Zac is one of the junglers I go to when I want to carry and have a ton of presence on the map. If you want a jungler, you can't go wrong with him. Same goes for Thresh as a support. Personally, I'm not much of a support player so I'd go Zac.