While answering questions for BuzzFeed Brews, Daniel Radcliffe answered the question of "Which franchise would you do?" with "Batman." When he realized that they are already rebooting the character, he offered himself up for the role of Robin.
The exchange happens around the two-minute mark, and Radcliffe comments that he'd be "perfect" for Robin. Honestly, yeah, he does fit that character best. If he was going to enter the world of superheroes, I can't think of a character that would work better.