You can't go wrong with J.K.. He's one of those actors born to play the role.
Speaking of which, that's an interesting discussion. I don't really consider Sam Jackson "born to play" Nick Fury since the Ultimate character was modeled off of him, but what actors have fit their roles perfectly? I would say Patrick Stewart, Robert Downey Jr., and J.K. Simmons are absolutely perfect castings. Chris Evans, while having a very young face and spirit, brings a lot of honesty to the role of Captain America. On the same vein, Hugo Weaving was the perfect Red Skull (still sad that he won't be coming back), and Tom Hiddleston IS Loki. I'd even give Anthony Mackie a respectful nod, although admittedly I am not completely familiar with the Falcon.
Outside of that, however, I can't think of many other actors I've fallen in love with.