Concerning this question of best actor cast as superhero, I do concur that RDJ as Stark and Chris Reeve as Superman are the top two. Only thing is I can't quite say for sure which of the two is the better fit. Chris had the build and I will say his Clark Kent was bang on. Supes though, I don't quite know as to me, his voice maybe still seemed a tick on the boyish side, even when he used a deeper voice. You could hear it still sounded a little like pleading when he would shout "Don't do it, the people!"
The reason why I say that is before Superman the Movie came out, I grew up on some of the cartoons and audio recordings of Superman and they tended to cast voice actors with a bit more masculine voice to do Superman. Reeve didn't entirely quite have that, although I admit by the end of the first film, I got that he WAS Superman as everything else was there with the look, the build (even if it was ever so slightly on the skinny side relative to how muscle bound DC was drawing him in the late 1970s) and the mannerisms.
For that reason, the voice I think was probably nailed perhaps a tick better by Brandon Routh in Superman Returns (if they had put a variation of the Reeve costume on him or the Dean Cain Lois and Clark one, which IMHO was probably the closest to the classic comic costume, Routh would have subplanted Reeve in my mind entirely). Dean Cain also did pretty good in Louis and Clark as I think his look was spot on as Superman, although maybe just a tick too confident as Clark (yet his interpretation was the post DC Crisis rebooted Superman, where Clark was the character and Superman was the disguise). Cain is also slightly let down by the voice as well, same as Reeve (just my personal opinion though).
Stark on the otherhand, RDJ seems to nail all the proper traits for an arrogant womaning, hard drinking, hard partying, wise cracking guy who happens to build weapons of mass destruction and can woo the most anti-war hot body journalist into bed with him. But when he turns on that Ironman seriousness, look out. You know SOMEBODY is going out in a body bag. So maybe RDJ as Stark/Ironman gets the nod over Reeve as Superman.
Just thinking about it as well, Toby Mcguire as Spiderman in the Raimi films also seems to be a case of perfect casting. McGuire has the innocent voice, yet it suits Peter Parker perfectly. Spidey tended to have a bit of a wisecracking mentality, similar to Stark, although usually the wise-cracking was in thought balloons for the reading audience as opposed to speaking them outloud for the other characters (but he did speak like that from time to time). McGuire doing the slightly weird Spidey quirks before he got the costume also seemed to fit. He felt like a "freakshow" and could keep with the brooding "with great power comes great responsibility" vibe for quite awhile.
And as for one final bit of left field "perfect casting"... how about Lou Ferrigno as The Hulk? Granted, one will never quite ever see a realistic actor only portrayal of the Hulk as in the comic, since comic Hulk can't really exist in reality (although I admit the CGI work in Marvel's Avengers has pretty much almost cracked it compared to what came before). But, if we were to see a big green muscle bound creature in a realistic setting (such as Kenneth Johnson's Hulk TV series), Ferrigno seemed to suit the part properly. I do admit though that the range of acting required wasn't all that much. Bill Bixby also seemed to play a pretty believeable Dr. Banner.