I think he has, what...? 30-45 minutes of screentime? If that. That makes him a supporting character. Much like Anthony Hopkins in The Silence of the Lambs. Like Tony, he just stole the show.
I think of Hopkins as the male lead of Silence of the Lambs. It certainly wasn't Anthony Head or even Scott Glenn. As for Ledger in Dark Knight (note to self, Dark Knight, not Dark Knight Returns
,) for how many of those minutes is he the center of the scene? Especially scenes without the official star? The number of minutes is a good starting point but Ledger was in a starring role, just as Hopkins was in Silence. Or Michael Keaton in Beetlejuice, even though he had maybe nineteen minutes on screen time. Does anybody else even remember Alec Baldwin was in Beetlejuice?
Much of the publicity campaign pushing for Dark Knight explicitly mentions the huge amount of money the movie made. That is a new low, even for Hollywood, in esthetic criteria.
PS Almost forgot---if you mean, who is the
star? Obviously you're right that it's Bale. If you don't admit the possibility of multiple leads, no matter how uncommon, then the question admits of no debate. But this merely seems unduly official, misleading conventional wisdom, to me.