All growed up and grouchy, I’m not big on comic book movies, but I was eleven when Spiderman was born and always liked the early 60’s Marvel comics.
With that as my reference frame, I thought casting Tobey Maguire as Spiderman was a stroke of genius.
This is the one for me. I collected Iron Man comics for almost two decades, and when RDJ appeared as Tony Stark, he just absolutely nailed it IMO.
Don’t know if she was ever a comic book character, but the part of Lara Croft was made for Angelina Jolie.
What? No votes for Jessica Alba as Susan Storm/Invisible Girl? 
Seriously, JK Simmons is spot-on - better than either of the modern Peter Parkers or Mary Janes, as far as casting. Chris Hemsworth has done pretty well with Thor, nor can I complain about Hiddleston’s Loki.
I mention this every time the subject comes up, but casting the hero is the easy part. Any reasonably fit person (and Hollywood has a few of those) can look good in the suit. What you need to do is to cast someone who can play the secret identity. Anybody can be Iron Man; Robert Downey Jr. was great as Tony Stark. Christopher Reeve was great as Clark Kent. Tobey Maguire was great as Peter Parker.
She was fine as Invisible Girl, but not as Sue Storm. She looked good in the suit, but I don’t think she was convincing as an astronaut. That’s why casting her didn’t work.
Richard Libertini as Geezil in Popeye. He was the perfect representatio of the character: it was almost as though Segar traveled forward in time, saw him, and said “That’ll be perfect!”
Shelley Duvall as Olive Oyl was almost as good.
I’ll second that nomination.
In a strange twist, I think Ryan Reynolds was a perfect choice for Wade Wilson/Deadpool, but the character itself got butchered beyond recognition in X-Men Origins: Wolverine. Too bad. He had the athletic physique, can pull off the serious, intense look, and still has good comedic delivery and timing.
I thought Tom Skerritt looked a lot more like the comic book Tony Stark. Also in disposition.
Another vote for Robert Downey Jr. Not because he’s the exact image of Tony Stark through the decades but because he made the part his own more completely than any other actor in a superhero movie.
To answer the OP, Chris Reeve was an ideal superman: tall, beefy, with a serene and confident demeanor; the kind who can stand squarely with his arms crossed and not shrink when face-to-face with anyone. Nicholas Cage was better off casted as Batman. Adam West and Michael Keaton were brilliant as Bruce Wayne but as the bat himself, I’d say Christian Bale. Among the Bruce/David Banners, none of them fit the bill. I was thinking more of guys like Matthew Broderick or Peter Mcnichol to play the pushover doctor. Linda Carter was perfect as Wonder Woman.
Oh yes, Daniel Radcliff as Harry Potter. Does that count?
Another vote for RDJr as Tony Stark. I wasn’t an Iron Man fan when I went to see Iron Man for the first time (I’d read a few of the comics, but he was always too “little boy’s idea of a rich playboy” to appeal to me.) I walked out of that movie a believer. I think more than anything else, his performance was what launched the Marvel Cinematic Universe.
As the spouse says, “They got Tony Stark right first and foremost, instead of trying to get Iron Man right.”
I also think Chris Evans was a good choice for Captain America. I didn’t like him as Johnny Storm, but he nails the “all American boy” vibe that’s essential for Cap.
Julie Newmar as Catwoman! All the others are just cheap im-purr-sonators.
RDJ nailed Tony Stark. So damn perfect. Marvel owes him every dime he charges, and he knows it, and they know it.
I think physically speaking, Chris Evans as Steve Rogers/Captain America was a really good choice. I’m pretty sure every person attracted to males lost their breath when he stepped out of that tube with his awe-inspiring nekkid sweaty chest. Likewise Hemsworth’s Thor. I can BELIEVE that body belongs to a thunder god from another realm. Yowza.
Character-wise, I’d like to see more of Ruffalo’s Banner/Hulk, and I never ever in a million years thought anything could get me to think of the Hulk as an interesting character, so that has to be a compliment to the actor/director.
The times they were actually able to get fanboy wishlist castings to happen-
Patrick Stewart at Prof X
Michael Chiklis as the Thing (he WAS the Thing)
Ron Perlman as Hellboy
Tom Hiddleston as Loki knocked it out of the park and into the stratosphere.
Jack Nicholson made a nifty Joker, IMHO.
There have been some great ones lately, but I’m going to go with Christopher Reeve. People remember Superman as being a great movie, and it was… But almost all of its greatness was riding on the shoulders of the superb job Reeve did. And Superman is a particularly challenging role to get right, because of his lack of a mask: The actor needs to present Supes and Clark differently enough that even Lois Lane won’t notice the resemblance, while still making both of them completely believable.
Christian Bale portrayed my namesake the best in Live-Action, but he can’t hold a candle to the Batman from the animated series.
RDJ reinvented Tony Stark which is great but much different from “perfect fit” spirit of the OP. Tony Stark of the page was never anywhere near RDJ’s performance before the movies. Don’t get me wrong, RDJ’s Stark is better, but it’s outside the mold.
J.K. Simmons /thread. Honestly, perfect fit. Honorable mentions to Hiddleston as Loki and Chris Evan’s Cap.
As someone noted, RDJ fits because he got Tony Stark right, and his Tony Stark does fit with my memories of Iron Man during the Dave Michelinie/Bob Layton era that is when the title actually found its footing. In large part, that is when - as someone in the letters pages at the time called it - the title became “The Adventures of Rich Man” and entire issues went by without him wearing the armor.