I like that it is phrased as your "favorite" as opposed to "best" because for me they are not the same.
While I am impressed with Jack Lemmon's ability to cover all the bases, I do not think he is the best at the craft.
I also like Tom Cruise, Clint Eastwood, Tom Hanks and Gene Hackman but there is no actor that I can recall that can transform himself and completely lose his own persona like Daniel Day-Lewis. He and Gary Oldman have that ability.