Connery was a director’s choice which, I suppose, is what counts. But Ian Fleming’s ideal James Bond was David Niven.
James Garner as Jim Rockford
Zachary Quinto as Leonard Nimoy as Mr. Spock.
Adam Baldwin will never have a better role than Jayne, and Nathan Fillion will never have a better role than Malcolm Reynolds. They had great characters that gave them a lot of room to work: dark & ruthless, one seemingly by nature(or perhaps because of an unfortunate upbringing), the other due to surviving a savage war, both struggling with their nature, wanting to be more, to be better. They made the most of the opportunity. Of course the whole show was cast to perfection, but I don’t want to give, “Everyone in the cast of Firefly” as an answer.
Maybe I’m missing the point of this thread, but it seems to me that most of the posters here are missing the point (unless I am, if you take my meaning).
I don’t think the point is, what casting choices turned out to be ideal. That’s a matter of the actor taking the role and making it their own. It’s perfect in hindsight, in other words. That’s a good thing, of course, but not terribly interesting.
A more interesting concept, and what I thought the OP was getting at, was to come up with new, unused casting choices that totally utilize an actors potential. Much like **the_diego **did in his post. I’d find that much more fascinating of a conversation.
Thank you.
Tim Curry as The Darkness in Legend acts through about forty pounds of makeup and is by far the most memorable and intense performance in the movie, and one of the greatest bad guys in movie history. They go SO LUCKY finding Curry for this part. The way he talks (and I don’t just mean the artificial lowering of his voice) is so grandiose and threatening, but at the same time Curry has this pathos that is irresistible.
OK, here’s one for you, though it’s purely hypothetical (the time when it might have been filmed is long past): Going after Cacciato with Michael J Pollard in the title role, Richard Widmark as the burned-out overage Lieutenant, and Sam Bottoms (the squid with the mustache in Apocalypse Now) as Stink.
That would have been a great movie! 
James Cagney said that he was met a drunken John Barrymore (was Drew’s grandfather ever not drunk?) who insisted that Cagney was tailor-made for “Playboy of the Western World”.
Marshall Crenshaw was cast as Buddy Holly in “La Bamba”.
William Talman as District Attorney Hamilton Burger in “Perry Mason”. As Erle Stanley Gardner noted, even though you know he is going to lose (well,almost always), Talman just had this barely contained glee that he thought he was going to win.
Dennis Quaid as Hal Jordan.
Will Smith turned down the ‘Neo’ role in The Matrix, and Keanu Reeves got it. I cannot imagine anyone else playing Neo and the movie wouldn’t have been such a big hit without him. IMO.
I can imagine Brandon Lee playing Neo.
I know that this thread is for actors who haven’t been cast in their perfect roles yet, but nobody did George W Bush better than Timothy Bottoms.
Euguene Levy would make a great Vince Lombardi.
If they ever make a movie based on Asimov’s Foundation novels, Adrian Brody should play The Mule.
Brian Dennehy as Martin Luther. Just because of the physical resemblance. Of course, he’s too old now to play anything but very-old Luther.
Back in the day (the 80s or so), I thought that a filming of Keith Laumer’s *Retief *stories would have been perfect with Tom Selleck as Retief and John Hillerman as Magnan. Of course they are both far too old by now.
I also thought Pat Hingle and Nick Nolte would make a good father & son pair, if a movie needed a scraggly-looking, gravely-voiced father and son.
Everyone in Lord of the Rings. Now, much of the dialogue and scenes that deviated from the book I didn’t like, but everyone was perfectly cast IMO.
Also Jim Carrey WAS Andy Kaufman in Man in the Moon
Bruce Campbell as the gym coach at a high school for young superheroes in Sky High. Especially since his superpower was shouting really loud.
Meryl Streep was BORN to portray Julia Child…Oh wait.
She made a better Julia Child than Julia herself.
With the exception of Denethor.