All have really good voices, but Ryeowook IMO is the best. He has great technique and he has a good ear so he rarely sounds off-key. He also sounds pretty much the same live. Plus, his voice is so beautiful :) Yesung is probably 2nd. He is really great at putting emotion into his voice, and his voice is nice and husky and deep. He just isn't as stable live and he can sound breathless. Kyuhyun is also good, but his voice doesn't stand out as much compared to Ryeowook and Yesung. His range doesn't seem as wide either. But they're all great.