The Seventh Doctor, Sylvester McCoy. An eccentric manipulative imp and chess master that views the universe in a vastly different way to us humans. A truly alien outsider played by a wonderful character actor, who wouldn't fit the traditional leading man role in any other show.
After him I'd rank Tom, Bill, Pat, and Colin as my other favourites. I like all the actors who have played the part, I just prefer certain interpretations over others.