I have been pondering this quite awhile and, after re-watching some episodes and reading other people's ideas, I have come to the conclusion that Amon is a tag team of Tenzin and Kya, his sister. Tenzin is the only one with the ability and the motivation to do this. He is the closest to Aang, the only known person to have practiced energybending and the most likely canidate for the avatar to teach. He also was clearly fed up with how the city was being run by corrupt officials and gangsters (such as Tarrlok and Lightning Bolt Zolt). He also most likely observed the schism in the city between the benders and non-benders, as evidenced in the show many times. What better way to clean up the city and also take out the most important criminals than to masquerade as an anti-bending leader? By performing this very Ozymandias-like move, Tenzin both eliminates the enemies of the city and unites the people against a common enemy (ends justify the means/ self-sacrifice/ etc). Therefore, Tenzin wants Amon to be defeated by the Avatar, and is saving the battle for the very end. In Tenzin's eyes, this would not be a "bad" deed, and might even function to train the avatar for future battles. Now let's move to hard evidence and why Kya must be incorporated into this theory. First, Amon is very often seen dodging attacks in a style very similar to airbending basic forms. Secondly, often when Tenzin disappears Amon appears. Take for example the episode "Turning the Tides." Tenzin leaves the air temple and leaves Lin in charge, during this time we see amon, and then Tenzin returns. When Lin is captured it is clear that she is taken to Amon later, due to Tenzin being on the bison at the time. When the chi-blocker reports to Amon "Tenzin has escaped, again." it makes one wonder why Amon doesn't ever confront Tenzin himself. There are many more instances such as this one, or ones in which Tenzin becomes aware of something at the same time as Tenzin, such as the location of Tarrlok. Now, this theory has two holes. First- Amon and Tenzin are both together in the stadium during the championship, and second - Amon is immune to bloodbending. Clearly Tenzin cannot be Amon at all times, and would need someone to lead in his absence. What better choice than this sister Kya, a powerful waterbender that was most likely trained by Katara, someone who has successfully resisted bloodbending. It is likely that since Kya is also a bender child of Aang, she also knows (or inherited) the ability to energybend. This, I believe, is the theory with the least speculation and the most solid backing available.
Two questions have been raised on this Wiki already (as I posted this in the wrong place like a COMPLETE FOOL).
1). Amon's Voice. Answer: Easily changeable within the mask. Clearly they have the technology to do this.
2). Amon does not have boobs. Answer: Amon's clothes are pretty baggy and heavy looking, I doubt you could see them through that, espcially if they were being concealed in some way like being wrapped up. Plus we don't know Kya's bra size. I am open to another proposition to solve the two problems with my theory other than Kya, but I think she is the most logical choice.
1. TENZIN WOULD NEVER BE EVIL OMGOMGOMGIDJKSFSDJK: Answer: Good and evil are useless and subjective discursive formations. Again to an Ozy callback - was he "bad" for what he did?