1. Amadeus: Mozart takes Solieri's composed music to a whole nutha' levah. It's more of a creative duel, but Tom Hulce busts out his learned chops on the classic harpsichord.
2. Scott Joplin: I had honestly never seen this early biopic starring the great Billy Dee Wiliams. The playing is coming in hot, but the pantomimed performance is lacking a bit in my perspective.
3. Legend of 1900: This proper movie duel takes place between the protagonist (1900) and Jelly Roll Morton. The facial expressions are pricessless.
4. Who Framed Roger Rabbit: The duel between Daffy Duck and Bugs Bunny was so memorable in their inevitable "this means war" jam-off. Kudos to the SFX wizards in getting those pianos to play on their own.