Carlos Moya and it isn't even close. His backhand was just awful. His solution was to just not hit any backhands
He made up for it by having a top 10 serve and a top 3 forehand, possibly even better than Federer or Nadal. Ridiculous forehand.
Great skill too, although he didn't use it that often. I remember Murray tried to do his cat and mouse thing with Moya and Moya just made Murray look stupid. Same thing happened with Stepanek around the same time.