Hideo Itami or Sin Cara? Both men were signed due to their international reputation and stardom in their respective countries. Sin Cara was the first signing under the Triple H regime and was supposed to be Rey Mysterios successor. And Itami, they brought Hulk freaking Hogan to Japan to announce his signing.
I'm gonna go with Sin Cara, because as noted he was supposed to be a big crossover star and succeed Rey. Itami was intended to pop the Japanese market, but I don't think anyone seriously thought he would be a huge star in North America. Too old, too much of a language barrier, too many injuries. He's been a bit of a disappointment, yeah, but he's already done everything in the business he needed to. Sin Cara not only flopped, but got injured numerous times and pretty much destroyed his own legacy in the process.