Simple question here. For me it's got to be John Morrison. This is because honestly, he's got the ballance of being a high flier just right. To me, Evan Bourne's got ntohing outside of his high risk offence. That's fine, he's very good at it but it does limit him as a performer. On the other hand Justin Gabriel's almost got too much outside his high risk moves to call him a high flier. He flies, but it doesn't make up the core of his offence. Especially when he's in control, when he's just as likely to do a moonsault as he is a takedown transitioned to a submission hold. That leaves him and Rey, and at this stage Morrison's better at high flying than Rey. His spots are more unique, and God damn I hate the 619 at times.
Sin Cara is excluded from discussion (and the poll, obviously) as he's yet to debut and as such it wouldn't be fair to include him when not every WWE fan's watched him perform, and even fewer have seen him go as many times as they have the guys in the poll.