A good hero is nothing without his or her villain to go up against, and a villain is nothing without a great actor portraying them. Nowhere is that fact more evident than in comic book movies.
Heath Ledger's casting as The Joker may have initially caused some outrage online, but his performance transcended the genre and turned The Dark Knight into something truly special. Much the same could be said for Jack Nicholson's take on the Clown Prince of Crime some 29-years prior as well.
Indeed, comic book villains have always held more appeal than their heroic counterparts for Hollywood's top talent. They're more colourful, more twisted, and have darker propositions befitting of serious psychological analysis, and therefore qualify as "proper acting". What's more, villains tend to only stick around for one movie, so they're perfect for serious thesps to cash in on without selling out to Hollywood for extended periods of time.
Oscar-winners like Gene Hackman, Tommy Lee Jones, and Ben Kingsley have taken on comic book villains over the past few decades, but the list of stars who've almost played them is perhaps even more impressive.