Too many, really.
Actors, Christopher Lee, by a long shot, but I also have a huge soft spot for vastly underrated character actors like John Lithgow & Rene Auberjonois (the latter actually having suffered by playing Odo, compared to what he can really do). I also like the following
Jack Lemmon
Cary Grant
James Stewart
Sir Alec Guinness
- in fact many of the classic actors. Currently, George Clooney & (at times) Russell Crowe are the only two who come close to that old school adaptability of acting. I also like genre actors like Ron Perlman, who have made a living out of playing below the radar characters in movies.
Actresses (again, I'm very old school with my tastes)
Julie Christie
Shirley MacLaine
Katherine Hepburn
Deborah Kerr
Modern actresses - not much to choose from, but Julia Roberts would certainly have had a lot better roles back in the 60's. Helen Mirren, of course, is a national institution. Cate Blanchett showed with that pretentious Dylan flick she's adaptable & talented as any out there. Imelda Staunton is a wonderfully kooky bird, who deserves better parts.
Other than that, it's the fit ones I like!
Why pick a definitive favourite when you have so much quality to choose from?