Beethoven. Toward the end of his life, he was outgrowing the traditional classical symphonic and chamber forms of his time. With the various instruments, both acoustic and electronic, available today for him to play with, and the additional genres for him to hear (I'm assuming we could fix his hearing too), I think the results would be awesome.
In rock, while there are many fine choices already mentioned, I'm with Jaybo, I miss the work of Clarence White.
