I would officially like to nominate Philip Seymour Hoffman into the category of instantly adds class to any movie he's in. This is a very distinguished talent held by only a select few. (Please note that that the word class can be interchanged with awesomeness.) So far the list is short as these men have already proven themselves worthy.
Morgan Freeman
Christopher Walken
Michael Caine
Alan Rickman
Ed Harris
In fact, these men may be the only reason we see the movie itself (example, did anyone really want to see Balls of Fury if it wasn't for Christopher Walken?) Though not officially nominated, I will closely be watching the careers of Christian Bale, Aaron Eckhart, Robert Downey Jr., and Willem DeFoe (he needs to atone for Spiderman though) and Gary Oldman. You notice how most were in Batman and The Dark Knight? $500 million in domestic box office tickets sales can't lie. (Unless the movie is Titanic. Then $500 million is a filthy liar that needs to shut its filthy mouth.)
Speaking of which, my classy female list is in the works. So far we have Kate Winslet. (Damn you sinking boat movie. You made a fool of me again!)