In this case my particular related example would be that Sampras is the most successful Men's Wimbledon champion because he won it most. He was the best at winning Wimbledon singles titles.
Whether or not he was better than Perry, Borg, Connors or Federer is irrelevant, you can only beat the guys in front of you. Of course with todays Racquets and intense training Federer (in his prime) would beat Fred Perry (in his prime) with his wooden raquet, more relaxed attitude to training and wearing the garments of the day but, in 60 years Roger Federer circa 2008 will look fairly pedestrian and amateur. Sampras never won the French Open Grand Slam, Lendle never won Wimbledon, does this exclude them from being called 'The Greatest' I wonder.
Lance Armstrong won most tours and, JUST IN MY OPINION, was the best of his generation (a short cycling generation lasting about 10 years). Lance was official World #1 before he got cancer remember.
MERCKX was the best of another generation, HINAULT another and so on. All you can do is beat the guys around you at the time. Of course the moment you are found guilty of doping your name is immediately made ineligible for the list of 'greats'.
If David Millar won the tour this year it wouldn't elevate him to a position any higher than 'Drugs Cheat' in my book.