It is extremely difficult to know who the best winner is, due to the fact that the term "best winner" is entirely subjective. There is a way to look at things more objectively, but it won't help us arrive at a clear cut answer.
Best Winner in terms of being the first to achieve the Title: Richard Hatch. He won in the first season, Survivor Borneo.
Best Winner in terms of Statistics: Sandra Diaz-Twine. She has won Survivor twice.
Best Winner in terms of Most Jury Votes: Earl Cole. He received 9 jury votes.
Best Winner in terms of playing "A Perfect Game": JT Thomas. He never received a vote at tribal council and got all possible jury votes (Seven).
Each winner has some sort of uniqueness to them where they can be argued as "The Best Winner".
This barely scratches the surface.
It can also be argued (despite the mention of some of them prior to this) that JT Thomas, Tom Westman, Parvati Shallow, Ethan Zohn, Richard Hatch, Tina Wesson, and Jenna Morasca are NOT the best winner because they have already lost once in another season, bringing them to a 50% Win/Loss ratio. Again, this is ONLY under the assumption that the best winner is based on the Win/Loss ratio.
Final Conclusion: There is no way to determine who is the "Best Winner".
Previous Answer: Sandra, Fabio, or Boston Rob.