Who do you think was the biggest bust that came over from TNA/Impact Wrestling to the WWE? Just going from previous work and expectations I'd have to say Austin Aries, although Low-Ki/"Kaval" would be a close 2nd.
Gotta be Low Ki. Fan voting pushed him to win NXT against the wishes of management and then they completely jobbed him out.
Bonus answer might be James Storm, who showed up in current version NXT for one show, got over, and then disappeared again.