I'd say Bob Sapp who became a megastar in Japan. He was under developmental contract in WCW until the buyout. He was voted the "best box office draw" by Dave Meltzer's Wrestling Observer Newsletter in 2002 & 2003. Bob Sapp vs. Akebono kickboxing fight in K1 drew 54 million viewers in 2003 - the biggest for a fight in Japan since Muhammad Ali vs. Antonio Inoki in 1976. He was also voted the "most charismatic" by the same publication in 2003 & won NJPW's IWGP Heavyweight Championship in 2004. He turned down a $1 million per year downside guaranteed contract from WWE in 2006 & 2007 because he would have been taking a drastic paycut from what he was earning in Japan (apparently Bill Goldberg also had a lot to do with that decision).
Wrestling Observer Newsletter (2nd November 2006): Bob Sapp met with Vince McMahon and Johnny Ace at the 10/15 WWE SuperShow in Los Angeles. His name has been brought up several times before, but this is the first time in which a serious deal was being offered. However, there are major roadblocks preventing the deal from being made. The main one being that he can make more money in MMA in Japan, not to mention that he gets movie work, and WWE likes to "own" their top stars. If the deal is made, it would show that WWE wanted him badly. It was said that there was a better than 50% chance of a deal being made after the meeting, but within the last week or so, its probably less than 50%.
Wrestling Observer Newsletter (18th December 2007): TNA is in negotiations with MMA star Bob Sapp. TNA has offered him an initial four-month deal, in which Sapp would be paid $2,500 per match. TNA wants Sapp to work a program with Kurt Angle, and then later with Samoa Joe. Sapp is paid about $35,000 per match for Hustle (he is under contract with Hustle for one year), and has received a seven figure offer from WWE. It appears WWE has been in talks with Sapp once again. Apparently, WWE and Sapp had serious talks a few months ago. There was talk that WWE would promote Sapp as a combined MMA/pro wrestling star.
After Bob Sapp it's basically for the most part all the Puroresu guys. I think Great Muta, Masahiro Chono & Kensuke Sasaki had the best chance to succeed in WWE as in my opinion they had the qualities to crossover & appeal to the American audience.