By what objective criteria can you judge "best"? There is simply no honest answer to your question.
The one who defined the role is William Hartnel. Without his charisma there would be no Doctor Who.
The most influential is probably Patrick Troughton who has influenced many subsequent doctors.
The actor who played The Doctor in the most episodes is Tom Baker.
Update:
I have redirected the question to what I think is a more appropriate category.
4
David Tennant and Matt smith
Tom Baker (in my opinion)
