I'd bring back Mozart long enough for him to hear Beethoven's 3rd Symphony. Had Mozart actually gotten to hear the Third Symphony it's possible that he might have gone on to compose something even better!
If Mozart couldn't make it, I'd fetch Wagner and take him to Hollywood to see all the Special Effects Labs. No telling what might have come out of that, but I'm willing to bet it would rival and probably best "anything" that's been done so far by Lucas, Spielberg and John Williams.
As far as modern Artists go, I'd have to have Caruso and Yma Sumac singing duets with modern recording equipment taking it all down.
I can think of a lot of others that would earn me the eternal gratitude of future generations 'til the end of time.
Best Regards,
TerryO