I would have to say that Alec Guinness is my favourite actor of all time, he managed to submerge himself so completely in a role that when you compared them, you could often not tell it was the same actor - this is the kind of acting that I admire the most.
His most famous roles are Probably Col Nicholson in '...River Kwai', Obi Wan Kenobi in the (good) Star Wars Trilogy and (the one I always assosciate him with the most) George Smiley in 'Tinker, Tailor...' and 'Smiley's People'. But my favourite Guinness performances were his early film ones - Herbert Pockett, Fagin and his classic roles in the Ealing films whilst his performance in 'The Prisoner' is a Tour De Force of dramatic acting. To be honest I can't single out one performance that I like the most - they are all so good. What are the favourites of other Guinness fans?