Why is Val Kilmer the best Batman? So many reasons - Batman Forever was sorta the perfect compromise between the goofy cheesiness of 60s Batman and the dark gothic tone of the Tim Burton Batman films, giving something for everyone - there's plenty of weird darkness if that's your thing, but there's also SO MUCH campy weirdness if you prefer stuff like Jim Carrey's Riddler or Tommy Lee Jones' complete disregard for his status as Oscar winner with his stupidly entertaining Two-Face. And at the center of it all was Val Kilmer, perfectly walking the tight rope between the two extremes at play.
But most of all, Val Kilmer's probably the actor who's the closest psychologically with Bruce Wayne. That is to say, he's got a lot of issues. Joel Schumacher - who thinks Val Kilmer was the best Batman ever - described him as "the most psychologically troubled human being I've ever worked with." And Joel Schumacher has worked with Nicolas Cage TWICE.
I mean, you have to be pretty nuts to put on a bat-costume and go around punching people who kinda look like penguins, so no wonder that the person best able to channel that nuttiness so easily was Val Kilmer.