This would be really hard for me to pick. While I watch all kind of movies, I've found that many of my favorite actors are not necessarily "good" actors. A childhood hero of mine is Arnold Schwarzenegger. I've seen all of his movies, most of them so many times that I can recount his lines, and he is easily one of my favorites. But he's not an "amazing" actor in the traditional sense, so I like him for other reasons. On the other hand, someone like Tom Hanks is a guy I admire for his talents, and I acknowledge that he is an amazing actor, but I wouldn't rank him very high personally, because he's not really a favorite of mine, and the kind of movies he is most known for, is not the kind of movies I love the most.
I think my current 4 favorites would be something like this:
Hugh Jackman (The Prestige, The Fountain, X-Men & sequels, Prisoners)
Kevin Spacey (House of Cards, Usual Suspects, American Beauty, Seven)
Robert De Niro (Taxi Driver, Goodfellas, The Godfather Part II, Heat)
Ryan Gosling (Drive, The Place Beyond the Pines, Only God Forgives, Blue Valentine)
And I had to leave out a few I really love, such as Tom Hardy, Christian Bale and Arnold. There's many others as well.