My 2 year old son likes to listen to Pink Floyd. At least it is the only band/genre/etc that he can listen to without doing anything else - just staying in place and listening.
I've tried playing him a variety of music, mainly based on my opinion of which music is good and which is not. I've tried playing lots of different old stuff - from rock'n'roll to classic rock to classical music - the only thing that caught his attention was a Pink Floyd "Best of" album.
I think the best music for a child is the one that he chooses himself. It is the only important criteria. If it's loud and hard, just turn down the volume. But that's my opinion.