Yikes, that's a hard choice. There's so many. Well, on top of my head, here they are:
Cate Blanchett (I love her graceful way of acting, even in movies like Indiana Jones)
Milla Jovovich (No one can beat Jovovich in a sci-fi action movie
Nicole Kidman (A great actress)
Now that's the hard choice, 'cause they're all unique (they are not in order of better; it's really impossible to say who's better).
Ian McKellen (should I add that his Magneto is in my top 5 best villain?)
Patrick Stewart (iconic presence and voice)
Robert Downey Jr. (I can't pin-point exactly why, he's just so unique)
Bruce Willis (my favorite action/comedy star; no one can beat him)
Johnny Depp (I know, I know, it's cheesy; but true. He's a great and unique actor, even in his more ordinary movies. And I bet no one could have
pulled off Sweeney Todd as good as Depp did)
Tom Hiddleston (no one should inherit the role of Loki except Hiddleston)
Alan Rickman (he's unique through his more theater-like acting. And anybody saw him in Galaxy Quest?
Hugh Jackman (a great actor)
Antonio Banderas (Okay, maybe Rodriguez's movies are crap, but they are great crap movies, and Banderas is always great in them)
Ralph Fiennes (marvelous as Voldemort and Hades)
Brent Spiner (Yep, I'm afraid Data will always be in my childhood memories)
That's it... I think.