Stewart once said he started going bald when he was nineteen, so all through his career, if a role requied hair, it was a wig or toupee.
Christopher Reeve as Superman. I grew up watching George Reeves on TV, but even a childhood memory couldn’t overcome the utter conviction Reeve brought to both the Clark Kent and Superman characters. It also helped that Reeve looked like he was drawn by Curt Swan.
Can you imagine Deadpool being played by anyone other than Ryan Reynolds? I can’t.
::cough, cough:: Jared Leto, Suicide Squad, ::cough::
I thought Steve Carrell did a pretty good job.
We haven’t mentioned many women in this thread, so I’ll highlight Audrey Tatou in Amelie, one of the most delightful, funny, and charming films I’ve ever seen. It’s one of my all-time favorites (I just happened to watch it again a few night ago) and she is just perfect in the role.
Take that back! Brian Lee as the Fake Undertaker nailed it! 
J/K, but at least he was better than the fake Diesel and Razor Ramon! 
It is impossible for me to picture “Han Solo” and not see Harrison Ford. He did a great job with that character, and deserves to be the movie star he’s become.
However…
Kurt Russell was under consideration for Han Solo. At the time, I think everyone knew him from a slew of Disney movies, and he had something of a clean-cut, boy-next-door charm to him. looking at IMDB, he was mostly doing TV guest spots, with a couple of steady roles on shows I’ve never heard of. On the one hand, he was a more established actor than Harrison Ford, and would have brought a little bit of name recognition to “Star Wars.” On the other hand, public perception might have worked against him. Would everyone accept a former Disney star as a rogue and a smuggler?
But then four years later, Russell hits the big screen as Snake Plissken, and the year after that he’s in “The Thing,” and from then on he’s able to have roles as legitimate bad-asses, as well as comedic roles and some good dramatic performances. I could make an argument that Jack Burton in “Big Trouble in Little China” is just a comedic take on Han Solo.
I think this is one of the more interesting “what-ifs” in movie casting. I’m a fan of Kurt Russell’s, so I’m probably too biased, but in my opinion he could have done an excellent job. He would have to have been the kind of actor that could do a lot of his own direction, as Lucas is famously not good at directing actors, but something tells me he could have been quite good.
The Han Solo role has to have someone with the right kind of charisma. Not just handsome and rogueish looking, but the right sort of smart-assed wisecracking sang froid.
Harrison Ford has it, Kurt Russell has it, as evidenced in “Big Trouble in Little China”. Tom Selleck, while possessing the looks and a lot of charm, doesn’t really have that same smart-ass charm. Thomas Magnum wasn’t quite the smart-ass that Solo is- he’s definitely kind of roguish, but not the same kind of rogue.
Ryan Reynolds could probably do it, if he toned it down a bit- Deadpool is a bit TOO smart-assey
Jeff Bridges as The Dude. IMO, this is in the others-could-do-it-but-not-as-well category, and maybe a little more than that.
David Suchet as Hercule Poirot
On this subject, I know when I heard they’d cast Elijah Wood as Frodo I said to myself, “Wow, that makes perfect sense.”
I thought Peter Ustinov played him well.
How about Robin Williams in Good Morning Vietnam, which I thought really took advantage of his ad libbing with the DJing scenes? Same, too, with his role as the Genie in Aladdin.
Honestly, I think in both cases Robin Williams did his thing, and they built the film around it. I think I read that’s how they animated the genie in Aladdin; he did the vocal performance and they animated to fit.
Irene Ryan as Daisy Moses (“Granny”). Many actresses auditioned for the role, inclding Bea Benaderet (eventually cast as “Cousin Pearl”), but the seach ended when Irene read for it.
To be fair, I really liked what Harry Hadden-Paton did with the role in last year’s Lincoln Center revival. His Higgins was a petulant man-child, which made Eliza a stronger character.
When an actor writes a role intending to play the part themselves, it’s probably easy (and fun) to make that role theirs alone. John Cleese wrote and described Basil Fawlty as a “brilliantined stick insect” because that described himself exactly as the character he envisioned. I challenge any other actor to fit that description and create that character so perfectly. There can only be one Basil Fawlty. 
Now if we’re talking about roles not written with a specific actor in mind but that ended up fitting someone like the proverbial glove to the point of where I can never see anyone else in that role - Katharine Hepburn as Eleanor of Aquitaine in The Lion In Winter. Absobloodyfrickinlutely PERFECT.
Well, not really. It’s always bothered me that the whole point of sending something humanoid instead of just an overt killbot is to blend in, and yet they chose a body shape that only a handful of humans worldwide have.
(I think the fanwank is that the android skeleton is chunky enough to need a big body – but then we can ask, why not make it look like an obese human?)
I like Schwarzeneggar, and think his character and imposing figure helped sell the film of course, but I’m not sure if that’s the same thing as saying that he’s perfect for that role.
It’s not hard to think of movies that would have sold better with Arnie at the helm, but made less sense.
I’ll follow that with:
Toni Collette, “Muriel’s Wedding”
Charlize Theron, “Monster”
Nobody’s mentioned Jack Nicholson yet! His play in One Flew Over the Cuckoo’s Nest was special. Some of his other roles also qualify I think.
Some others come to mind, but I’m stymied by my lack of imagination to see others in the role.
It’s not quite the same thing, but Nick Nolte was special in The Good Thief, about which Roger Ebert wrote: “it is clear that [Nolte] was born to play Bob. It is one of those performances that flows unhindered from an actor’s deepest instincts.”
Jensen Ackles as Dean Winchester.
Tom Selleck as Magnum.
And, frankly, Edward Woodward as The Equalizer.