What makes a "great" actor?

Lissener and sirtonyh, even if we don’t agree on nuancees, I feel we are on to something common. I very much agree on Aniston, who’s the most under appreciated actress of her generation.
Your list, lisener, added some names I thought of, and some I didn’t even remember, but agree with.
Especially Bill Murray. Seeing him in ‘The Royal Tennenbaums’ it’s quite obvious that he has a range few produers have credited him with. He’s young enough for me to think that he’ll go on to surprise a lot of people/win awards/finally get recognition.

I’ll have to disagree about Meryl Streep. I think the best performance by an actor on screen ever was her performance in Sophie’s Choice. Compare that character with her role in Out of Africa.

One of the best actors to watch in action was Spencer Tracey. I could never figure out how he did it. Just couldn’t catch him acting.

Listening is the first step.

Billy Bob Thornton could play the role of the Shenandoah Valley and be convincing. There is nothing he can’t do when he makes up his mind to it.

And due to his cuteness Cruise has won 3 of the 5 Golden Globes he has been nominated for and earned 3 Oscar nominations.