It sounds like an easy question: by which I mean that I expected to have a strong and somewhat inflexible opinion about the correct answer. But - no answer springs to mind.
Who lights up every scene he or she is in? Who transforms the words he or she delivers into poetry? Who can turn his or her hand to any role and enhance it?
Sean Penn is very highly thought of at present isn't he? I find Kenneth Branagh's delivery of Shakespeare to be better than anyone else acting nowadays. And - I know you'll think this my hormones talking - but I think Russell Crowe is a surpassing actor. I think that's why he is so widely fancied, although he isn't handsome.
Of the three, probably Kenneth B. Ah, I knew I could achieve dogmatism if I tried.