Chris Reeve as Superman, hands down. Yeah, it's nostalgia, but also I think Reeve captured the charm and humour of Supes, while still being strong.
I liked Henry C as Supes, but his was a more down to Earth Supes - I loved the characterization from MOS, but Cavill didn't manage to be as captivating as the last son of Krypton in his first outing as Reeve was. Hell, even in 3 sequels 2 of which were crap, Reeve was still great. So I liked Cavill, and I'm looking forward to seeing him in B v S, but Reeve was something truly special.
Maybe the thing about Batman and the Joker is that both of them have been played by a few actors, so it's not hard to think of them.
I guess that could also be said of Superman ( Reeves, Reeve, Cain, Welling, Routh and Cavill) but for some reason Reeve always comes to mind first.
So I can actually see other people as Superman, but when I think of him the first person I see is Reeve.
RDJ would be first runner up. I mean his Tony Stark is reasonably like the character in the comics, but when people think of Iron Man, they probably think of RDJ first - the same is true for Loki and Thor, they aren't necessarily like their comic book versions but have really created the roles on the big screen.
Actually, I'll put Sir Pat Stewart as tied with RDJ for runner up - as he is quite similar to how I always imagined Charles Xavier to be, and just has to show up to get the look right. I'd have to say McKellen's Magneto is the same, in that first X-movie he and Stewart were the best things by far.
Honourable mention to Bale (captured the many faces of Batman/Bruce Wayne and the rage, which is something that hasn't really been done well before). Same goes for JEH as Rorschach, now that was a truly inspired bit of casting, which made up for the very average casting for the rest of the cast of Watchmen.
Tobey Maguire also gets a mention as Spidey.