Favorite band is a LOT harder. There are a lot that are neck-and-neck in the top slot depending on my mood that day.
I guess if I look at it from the perspective of "You're only ever going to get to listen to one band for the rest of your life, which one do you choose?", I'm gonna go with Metallica. But that's like asking me to choose my favorite kid.
Well, harder actually. I know who my favorite kid is.