The Flash fans have been teased by the question: "Who is Zoom?" all season long. After this week's episode it seems we are getting closer and closer to the answer. In fact one could say that we can already guess the Identity with quite much certitude.
The first clue, that we've been given a few episodes ago is Hunter Zolomon. We were shown that he exists and what he looks like: he is the Earth 1 version of Jay Garrick. If you don't know(which I doubt), in the comics Hunter Zolomon is the man going by the name Zoom and he is a former friend of Wally West(the current Flash, as Barry was dead at that point) who got angry with the Flash for not wanting to travel through time to prevent him from being paralyzed.
The comics' Zoom had more in common with last season's Reverse Flash(the wheelchair and the paralysis, wanting the Flash to become better, the color of the suit, etc). Which is why the show's Zoom sports a different outfit, and probably a different origin story(as he might not know either Barry or Wally).
Well, I haven't started with Hunter Zolomon randomly. We knew from the beginning that it would be a familiar face under the mask. But we all expected it to be someone we knew from the last season, or more likely their Earth 2 counterpart. And a faction of the fans have been trying to come up with theories that would make Wally West of Earth 2 be Zoom. Some people said it could be Earth 2 Barry(which they've been proven wrong last week). Others have indicated the Earth 1 Eddie or Ronnie or Earth 2 Joe, and the more plausible for a long period of time: Earth 2 Henry Allen.
Now, I won't be spending much time saying why the last mentions can't be Zoom, but I'll go briefly through it. Eddie and Ronnie are both too short(1.80m/5'11'' and respectively 1.83/6') compared too Zoom who is taller than Barry(Grant is 1.88m/6'2''). And so is Wally(1.73/5'8''). Even if it wasn't for height, Eddie is surely dead, as he died before being sucked, otherwise, Eobard Thawne wouldn't have been erased from existence, time finding a way to rewrite itself leading to Thawne's existence(making him probably a resident of earth 2). Earth 2 Eddie, would have been mentioned or shown if it was the case. But however, Rick Cosnett is busy with Qunatico, his new TV Show, which prevents him from reprising his role for more than at most an episode, which would not be enough. Ronnie of Earth 2 is dead and probably so is his Earth 1 counterpart, otherwise he would have shown in this week or the last's week episode and would have been taken home. Or he is probably on Earth 3 or 4 or n(it was never specified that the singularity was leading to only one parallel earth). And I don't buy the theory according to which, Zoom could be any of these three, but he looks like this because he is buffed up by the speed. If it was the case, this phenomenon would've been mentioned in the show. Plus there's no mention that Zoom has an alternative speed source than Barry(the man in the mask, if he was a speedster, looks dried of his speed, otherwise, he would've escaped) and even if he had, he can't take it himself. He needs Wells' to extract it from Barry, so most likely he had not eaten in a long time, which would've made him revert to his original form, which is the one that we know(like 1.92m/6'3 1/2'' tall and beefy).
About Joe. Well, it is simply physical. Jesse L. Martin is just 2cm or ~1in taller than Grant Gustin(not visible, while Zoom is visibly taller; check this week's episode towards the end when Zoom threatens Barry) and he is not in shape. He is a little fat, while Zoom has a sculpted physique.
Speaking of Henry Allen. Many fans, including me, first thought he would be a very good possibility. Well that was until we saw that Zoom is visibly taller(like 1.92m/6'3 1/2'') than Barry, and knowing that John Wesley Shipp is visibly shorter(1.85cm/6'1'') than Grant Gustin, changed at least my opinion, cause it looks like Zoom is at least 8-9 cm(2.5-3in) taller than Henry Allen, and it is a big difference. Plus, when Barry talked on the phone with his Earth 2 counterpart's mother, it was implied that she was going with Henry in Atlantis(and it makes me think they have a normal life, and Henry doesn't have the motive).
And finally, my nomination: Hunter Zolomon(Jay Garrick's Earth 1 counter part). The first reason implies, of course, Zoom's physical traits which seem to be identical to Teddy Sears' traits(the only thing that makes me restless is that the guy in the suit in the pictures taken on set, had brown eyes, while Teddy has blue eyes). First Zoom's height that you can deduce from this week's episode when Zoom stands face to face with Barry on the other side of the glass. He looks like 3-4cm(1.5-2in) taller than Barry. And that's exactly the difference of height between Teddy and Grant. Next would be Zoom's sculpted body. Only one actor in the cast is both visibly taller than Grant and possesing the corresponding body shape.
Another reason is, that I believe that the guy in the mask in Zoom's lair is actually Jay(He is not Wally, because he has blonde hair, and because he is taller than Barry - check the picture above). And you are going to ask me: if that's Jay, who has been with the team so far? Here comes the perfect opportunity for the producers to tease the fans a little with the likes of the Rival or Golden Age Reverse Flash(who could be a clone instead of an android). Any of them would be working for Zoom, maybe involuntarily, or unknowingly. This would give him reason to take the impostor back for both probably the Velocity 9 and the divulging of compromising information regarding Zoom. What information? Barry would've surely told Jay about the man in the mask and it would've certainly lead to discovering information that does't fit with what Jay knew(or if he knew, what would he have told them). The Jay impostor might've as well never been a speedster(remember that when Caitlin ran tests on Jay, they didn't reveal anything specific to a speedster in his body), which gives him the reason to trick Caitlin into creating Velocity 9. Jay's disease could actually be a disease of the of the impostor and namely the Rival's - cause he tried to replicate Jay's powers with, and pay attention: VELOCITY 9 in the comics and probably VELOCITY 6 in the show, which would give him reason to bargain with Zoom to send him as a spy in both of their interests(or go there by himself and ultimately posing as a threat to Zoom), his being to create a stable form of Velocity with the help of Caitlin Snow. And if you ask me about appearance. Well, it could be plastic surgery, or the trick Eobard did with the real Earth 1 Wells', or Jay might not look like that at all, that being the face of the Rival, and Hunter Zolomon might be the Rival's doppleganger. And Jay? The real Jay could be the man in the mask.
Another reason for me to believe this is that the producers have preserved the identities of the characters as close to comics as they could. For example, last season, even though the man behind Reverse Flash's mask, proved to be named Harrison Wells, in the end his true identity was revealed as being his comic counterpart - Eobard Thawne. So this being said, I think that if they revealed the existence of Hunter Zolomon, they will use him for more than a scene of him sitting on a bench in the park. Plus the producers teased us that it was going to be a familiar face and not a new one. We've met that face in the first episode of the season, and we've also met Hunter(so it's going to be a familiar face under the mask, as the producers teased). Plus, the man lives in the neighborhood of the Star Labs and we're supposed to believe that he wasn't affected by the explosion?
With all these being said, I think It's safe to say that the chances are that Hunter Zolomon is Zoom. But it doesn't matter what name the man under Zoom's mask will have, he will be played by Teddy Sears.
EDIT: Just figured it out. Barry wishing to go back to earth 2 to save Jay and the masked man(who will prove to be the real Jay), will explain Supergirl/The Flash crossover, as he the team will not manage to calibrate the destination as Earth 2 from the first try, so Barry will first get to National City.
UPDATE: In episode 2x15 this theory is proved true. At least partially. But we'll have to wait to see about the rest of it.