First of all, lets get to the obvious clue. Which is that his name on earth 1 is Hunter Zolomon, who is Zoom in the comics. But I don't think Hunter is Zoom,afterall, why would a guy from earth 1 operate mostly on earth 2? I do however believe both Jay and Hunter are working together in order to manipulate Barry.
You might think 2 things rule out Garrick as a suspect: the fact that he doesn't appear to have any speedforce in his system and the flashback where we see both Zoom and Flash (Jay) fighting on earth 2. But heres where Hunter comes in, Jay/Zoom used DNA from his powerless doppleganger in order to fool Allen into thinking he has no powers. Maybe it was Zolomon who we first met in the end of the series premire. Why Hunter would want to help Jay? I don't know, it could be for any number of reasons. The other hole in my theory is that we've seen Jay and Zoom fight, abet in a Flashback, that was narrated by Jay (unreliable narrator). It is also possibe that he did the ol speed mirage trick in order to trick the citizens of earth 2 into thinking that there are two people fighting instead of one. Something Eobard used back in season 1 quite a few times.
But why would Jay, a seemingly good guy, be a closet supervillain? Well for starters he's dying and the only cure is more speed. What does Zoom want? Well for Barry to get faster so he can steal his speed of course! You'll notice early this season that Jay kept trying to get Barry to gain more speed first and confront Zoom last. But then earth 2 Harrison Wells comes along and tries to trap Zoom, instead trying to get Barry faster. We see this anger Jay, leading him to storm out. Zoom/Jay never shows up to the trap because team Flash already told him the plan. Zoom/Jay then decides Barry needs more motivation to get faster. So he shows up and beats him to a bloody pulp and then proceeds to cripple and humiliate him. All in order to get him back to increasing his speed.
So what do you guys think Zoom is?