Phew! I was wondering when someone was going to get around to mentioning Oldman. From Sid and Nancy to Dracula to The Professional to The Fifth Element? Get outta here … that’s superb range.
Props to Cadbury Angel for mentioning Alan Rickman. It took me many years to realize that he was the villain in Die Hard.
Cate Blanchette seems to me to deserve mention in this thread – I had to double-take upon seeing The Gift.
I agree that there’s a difference between “movie star” and “actor” although some can be both. Naturally, some actors are going to get typecast, but some get typecast because that’s all they can do, others get typecast because they do that one thing so well. Tom Cruise is an example of the former, Alan Rickman an example of the latter. (Although to be fair, I have avoided any Tom Cruise movies made in the last eight years or so, so maybe he’s improved since then.)
I’m not fond of Jack Nicholson, particularly since The Shining. Watching him now is like watching him impersonating Christian Slater impersonating Jack Nicholson. He makes my teeth itch.
I think character actors have a better shot at being judged fairly on their abilities, just because there are more roles they perform.
I think he can. When we saw him first, in What’s Eating Gilbert Grape? my husband commented that it was nice to see a such a challenged individual getting acting work 
I’ve seen him in a couple of bad roles, but I think he does his best to work within his character. I was pleasantly surprised by his recent * Catch Me If You Can* role. It seems he is growing into himself.
I agree 100% about Cate Blanchett and The Gift, but I want to ask a question.
What was Cate so good at doing in that movie? Were you impressed because she was so believable as a psychic? Was it that she was so believable as a single mother? That the Aussie portrayed a not-so-bright Southern white trash woman so well? I imagine the answer to all those examples would be yes, but think. What was at the heart of that film? Wasn’t it that her character Annie was truly terrified by the character of Donnie Barksdale, wife-beater and (she thinks) murderer? Don’t you feel terror for her? Don’t you desperately want her to be alright? Aren’t you on the edge of your seat because you think she’s truly in danger? If the answer is yes to any or all of those questions, then you MUST have thought that the actor who played Donnie Barksdale did a good job at playing a menacing, dangerous character. Right? Would Annie have seemed to be in as much danger if the character she was frightened of had been played by a dull, bad actor?
Bonus question: Who played Donnie Barksdale?
Keanu Reeves. That’s by far the best performance he’s given, but his average performance is…suboptimal.
Dicaprio was also very good in The Basketball Diaries. He seems hit and miss to me.
Phillip Seymour Hoffman has played essentially the same guy 3 times. His characters in Boogie Nights, Happiness and 25th Hour could be snapshots of the same (pathetic)person at different times in his life. He’s very good at playing that guy, though…
Another vote for Edward Norton, here.
Oh, also wanted to opine that Russell Crowe was better in The Insider than in A Beautiful Mind.
No list of mega talent has merit if it does not include Shelley Winters.
A Double Life
A Place In The Sun
The Chapman Report
A Patch Of Blue
Lolita …to name a few.