I can just see Robin Williams as Bobmbadil. Oh, My. Sorry, but that REALLY doesn't work. Come on, I really don't think Robin Williams could do it. He makes everything (I take that back, Almost Everything) into a comedy. Also, have you ever heard Robin Williams SING? I haven't. And knowing him, I think he wouldn't do a very good job at it.
I really don't know who would be able to Play him. Bill Murray maybe. Or Maybe not. I don't know what do you think?