Best: Leonardo DiCaprio, Tom Hanks, Dustin Hoffman, Julianne Moore, Reese Witherspoon, Emma Thompson.
That's based on diversity of roles and on their ability to make their character overshadow the person playing the role.
Favorites: Morgan Freeman, Clint Eastwood, Susan Sarandon, Meg Ryan.
Best movie role:
Reese Witherspoon in Freeway! (whoa!)
Susan Sarandon in Bull Durham
Clint Eastwood in either Gran Torino or The Unforgiven (or maybe even Bronco Billy)
Tim Robbins and Morgan Freeman in Shawshank Redemption
Well, if only looks count, I would say Salma Heyak or Beyoncé (even tho she is mostly a singer, but she has been in movies too)
Actors: Tom Cruise and Mel Gibson. Bad mouth both of them all you want, but both have great movies that I enjoy watching.
For actresses: Reese Witherspoon and Drew Barrymore top my list. Barrymore's movies generally suck (or aren't critically noteworthy), but her characters are always tender/innocent and that's what draws me to her.