I think it all depends on what you mean by a 'coup'. If you mean getting a serious actor for a role you'd never think they'd play, then these two would be my top picks. Probably Jack would be the number one, and Redford coming in at a close second.
And see, I don't consider any of those coups. For Downey and even Ledger, I remember some serious vocal dissent about them being cast. And in Downey's case, his battles with his personal demons had left his career in tatters. It was more of a coup for himself getting the role than the other way around.
As for the X-Men films- I don't think, at the time, that any of those actors were really anybody BIG. Anna Paquin had an Oscar, but that was years before. Patrick Stewart and Ian McKellan were established, respectable actors, but Stewart's biggest claim to fame was Star Trek: The Next Generation (or maybe Lifeforce
, and the only movie I remember seeing McKellan in before X-Men was Apt Pupil. Halle Berry wasn't a big name yet, and Hugh Jackman was an unknown. And pretty much everyone else were just working actors who weren't well known. And again, it was a coup for all of them to be cast in the movie for themselves, rather than the movie, as it helped launch almost all of them into superstardom.
For Nolan's movies, I think the biggest coup there was Michael Kane as Alfred. Pretty much all of the other actors (with the exception of Ledger, who did have a lot of mainstream buzz around him, and Liam Neeson, who is Liam f'ing Neeson) were actors known more for their small indie films than any major stars. And again, these movies helped to truly launch them into superstardom.
Ryan Reynolds is a tricky one. The biggest coup for him is that he was finally able to pressure Fox into green-lighting a Deadpool movie, which hit big. Before that, he was a TV actor that had some hits in rom-coms, and had a few forays into the super-hero genre that failed (and that he has since mocked mercilessly. That may be one of his best qualities- his ability to laugh at himself).
I'll give you Joaquin Phoenix. Definately a coup.