I couldn't picture anyone other than Charlton Heston playing George Taylor in the original 1968 version of 'Planet of the Apes'.
There are of course the movies that were or seemed specially written for an actor or actress. Think of The Mask and Jim Carrey.
From wikipedia: For the role of Lecter, Demme originally approached Sean Connery. Demme's first choice for the role of Starling was Michelle Pfeiffer Which film am I talking about? I think they got the two main characters dead right.
Danny DeVito could not do wrong when playing evil characters, e.g. in the movie Ruthless People (1986). Always gives me a Louis de Funès vibe.
Perfect casting, perfect performance, perfect film. Michael Biehn nails every facet of who Kyle Reese is.
Robert Walker Strangers on a Train Jimmy Stewart Vertigo Anthony Perkins Psycho Ingrid Bergman Notorious Ray Milland Dial M for Murder and of course Cary Grant for North by Northwest