When I think of actors that make it look easy, I think of Spencer Tracy and Gene Hackman.
I think a lot of people would be surprised if they watched a film of the real Patton. He didn’t look or sound at all like Scott’s portrayal.
Recentlty we began watching as many films as we could featuring one star or director. We gave up on Peck after watching a number of stinkers. Sure, a good actor can be in a lousy movie, but we saw nothing to come close to Mockingbird.
For me, the test is whether I can accept the role rather than simply seeing the actor. For example, John Wayne was always John Wayne first, rather than the character. Same way Kevin Costner is always Kevin Costner (whom no one would suggest is a great actor) walking through a role. I feel similarly (many would disagree with many of these choices) about Tom Hanks, Jimmy Stewart, Cary Grant, and Probably Bogart, Cagney, and Nicholson.
I note a number of folk suggest an isolated role or 2, as tho that shows “range.” Interesting. Raises the possibility that some actors might have been capable of greater range if given the opportunity, but instead were primarily typecast. So does some of that go to the actor’s choices?
I don’t believe Robert Duvall has been mentioned yet. I’d agree with folk like Lemmon, Tracy, Hepburn, Lancaster, Newman, Streep.
Also think the OP could benefit from some definition of “iconic supertars”.
Contrast Bogie in Casablanca or The Maltese Falcon with Bogie in A Quiet Place or Mutiny on the Bounty to see how brilliant he can be playing dissimilar roles. Check out pre The Shining Nicholson roles to see just how versital he could be (The Last Detail, Five EasyvPieces, Carnal Knowledge, The King of Marvin Gardens.)
Regarding Peck, have you watched Arabesque? I enjoyed it…its a thriller also starring Sophia Loren at her most stunning.
I was hoping to be the first to mention Hanks. He is clearly an iconic actor and has quietly assembled a staggering resume of magnificent performances.
If we’re doing some contemporary:
Kevin Spacey
Meryl Streep
Daniel Day-Lewis
Eddie Redmayne
Just keeping facts straight (bolding mine):
The Bogie films are “In a Lonely Place,” and “The Caine Mutiny,” (which was excellent).
Someone upthread argued that Daniel Day-Lewis shouldn’t count because he doesn’t really qualify as a heart-throb movie star, but I think he hit “sex symbol” status for a while around when LAST OF THE MOHICANS was a big deal in theaters.
Research? We don’t need no stinking research!! 
Agreed. The real Patton’s appearance and voice was somewhat underwhelming. If George C Scott led the 3rd and 7th Army, WWII would no doubt have concluded earlier. 
To my ears, the real Patton sounded a little like Walter Brennan.
I was thinking Carroll O’Connor.
Walter Brennan would have made a good general, too.
“Now listen here Little Luke, ya yella-bellied bastaad…I won’t have no cowaards workin’ on this here faarm…”
That was the last one we watched. Wife and I both thought it unwatchable.
“De Gaulle? He ain’t even in this war!”
Since we’re stuck with “Hollywood” actors, I would think that Kevin Spacey, Edward Norton and Gary Oldman should be considered. If we broadened it to include iconic character actors, M. Emmett Walsh was amazing. If we could break out of Hollywood, people like Helen Mirren, Alec Guinness and Peter O’Toole would make the list.
I can understand. Stanley Donan was experimenting with various FX and camara techniques and it does seem the story is something of an afterthought.
Without a definition, I’m forced to assume it means “my personal favorite indie-hipster cult film actors”. 
I’m starting to think maybe a list of iconic actors who can’t act may be shorter.