Michael Keaton.
As for why, that's best explained by this article:
http://gothamalleys.blogspot.ca/2011/01/batman-in-movies.html
"Again, I felt less is more with him in the sense of who he is. (
) Michaels eyes - it goes back to kind of like silent movie acting. I like when people sort of just look. Its a movie so you kinda get more between the lines then you do [from] the actual lines (
) There's a loneliness to that character and witheldness. Hes a character that is sad and is private"
"Even when he's standing there looking there's an electricity about him. Again this is why I wanted him for Batman because its all about that." (BR audio commentary)
"I wanted Michael from the start. I knew he could do it after working with him on Beetlejuice. And there is something in his eyes, a dimension of feeling, even with the mask on."(San Diego Tribune 1989)
"Michael has this explosive side. All you have to do is look in his eyes and you know he's nuts" (Newsweek, 1989)
Bob Kane: Michael Keaton has an edge about him. (...) [He] has a maniacal quality that Nicholson has, the same craziness going on in the eyes. (People, 1989)
Michael Keaton: What makes it doubly interesting is that he's kind of psychotic. At first, I wasn't willing to take it that far, but Tim was more than willing to take it that far. I read the script thinking, 'This guy's really angry and depressed and dark (Boston Globe 1989)