Iron man, easily. For multiple reasons:
1.) Joker has already been played by multiple actors, with multiple iconic performances. The public has accepted multiple actors playing the part.
2.) The Joker as a character and icon was already established in the public consciousness. This comes with its own challenges, but it has the benefit of guaranteeing that, no matter how iconic the performance, the concept of the Joker always exists outside any one actor. It’s viewed at in a light akin to famous theatre roles, like Hamlet. Sure, Brannaugh did a great Hamlet, but he still will never overshadow the role itself.
3.) On a related note, Iron Man was largely unknown to the general public before RDJ, and he was low B list (if we’re being generous) to comic readers. The Tony Stark in the MCU was created by RDJ, and is synonymous with RDJ, to the point that people are commenting on how they think RDJ is becoming Tony Stark with his new business ventures.
Whoever does try to step into the suit next (and I don’t think it’ll happen for a long while) they’ll have a much tougher time than any other actor taking a comic role. Even Wolverine. Though that’s the next closest.
