Andre Agassi, because he skipped the Australian Open until 1995, and then he won 4 Australian Opens, so for 8 years he skipped his best opportunity to win slams. Also he skipped Wimbledon for about 5 years. So instead of winning 8 slams he would have won 12+ slams. Don't forget, Agassi made the 1990 US Open Final and the 1990 and 1991 French Open Final, so he was definitely playing well enough to have won the Australian Open at least in those early 90s (maybe even late 80s) if he played there. Plus Agassi choked in his first 2 French Open Finals, should have won at least one of them. His lack of inner confidence cost him some big matches.
Drug cheats don't count champ.