It's hard for me to pick a favorite actor/actress or favorite movie, although my favorite movies tend to feature knockout performances by all involved.
One of those personal favorite films is LA Confidential, based on a sprawling noirish mess of a James Ellroy novel that was stripped down, souped up and populated by an ensemble of actors and actresses that were just perfect for their roles: Russell Crowe, Guy Pearce, Kim Basinger, Kevin Spacey, Danny de Vito and more. I've always enjoyed it, even after all the horrible stuff about Spacey's private life came out. Perfect cast; perfect acting; perfect pacing; perfect screenplay; perfect cinematography.
Another one is We Were Soldiers, with Mel Gibson, for much the same reasons. I've liked Mad Mel ever since Mad Max, but I thought this was a standout performance even by his standards. (And ditto Sam Elliott.)
While we're talking about Mad Mel: Add Braveheart to that list. Right at the top of that list, as a matter of fact.
Speaking of perfect films: the original Die Hard, which was the perfect action movie (and a pretty good Christmas movie, too). Again: perfectly cast, perfectly acted; perfectly paced, perfectly written and filmed. And with one of my favorite screen villains, Alan Rickman, stealing the show along with his tag-team partner and henchman Alexander Godunov.
I'll also nominate the late Rutger Hauer, in pretty much everything. Although he really needed a better agent, especially in the latter stages of his career (*koff*Hobo With A Shotgun*koff*).