Hmmm... This is a tough question, because there are so many great actors and actresses - each with their own quality that sets them apart. It's kind of like my top ten movies list, that contains well over 200 movies!
Off the top of my head though, a few of my favorites are:
Errol Flynn
Lon Chaney Jr.
Audrey Hepburn
Steve McQueen
Alec Guiness
George C. Scott
Cary Grant
Marlon Brando
Toshiro Mifune
Gerard Depardieu
Spencer Tracy
Jack Nicholson
Meryl Streep
Gregory Peck
Paul Newman
Jurgen Prochnow
Bill Murray (way under-rated)
Humphrey Bogart
Gene Tierney
Al Pacino
Robert DeNiro
For recent stars (last fifteen years or so) that really impress me, and will someday be in my "favorites" list:
Johnny Depp
Ed Norton
Brad Pitt
Benicio Del Toro
Pruitt Taylor Vince (he needs to get a bit more choosy in his roles though)
Tim Roth
Wow, I could go on forever!
I know I'm missing alot that I'll kick myself for later, but for brevity sake I'll stop here.
Don't even get me started on worst, or most over-rated actor/actresses! *cough*KevinCostner*cough*