Stefan Edberg -
Should have won the 1990 Australian Open when he retired to Lendl due to a stomach muscle injury, but had been playing an incredible tournament, dropping only one set up to the final, including total destruction of Mats Wilander in the semi-finals.
Also I believe he should have won the 1989 French Open against Chang when he was two points away from serving for the match in the 4th set, and let it slip away.
With those two wins, he would have had the career grand slam plus a total of 8 singles slam titles, forever putting to rest the "who is better - Wilander, Becker, Edberg" debate. I happen to already place Edberg as the best of those three, but most rate Becker or Wilander (or both) above him.