For me? Superman. Richard Donner and Tom Mankiewicz clearly understood and respected what made Superman the iconic character that he is. Plus, I'm not sure anybody will ever top Reeve's dual performance of Clark Kent/Superman. It's the standard to which pretty much all superhero performances are held.
Batman '89 on the other hand feels like Tim Burton had a very surface-level understanding of what makes Batman a great character (which isn't surprising since he's said numerous times that he's never read a Batman comic) and he clearly preferred spending time on the wacky villains in his two Batman films instead of trying to flesh-out Bruce Wayne.
I like Batman 89 just fine, there are three major things that hold that film together - Danny Elfman's score, Anton Furst's fantastic production design, and Nicholson's Joker. I feel like if you lose any one of those elements, the film loses a lot of its luster. Keaton is a good Batman, but to be fair he's not really asked to do much of anything aside from stand around and look cool. Maybe he'll toss a punch or a Batarang every now and then.
