Die Hard can't be better than BttF. Die Hard doesn't have Christopher Lloyd and Michael J. Fox. Instant fail.
This isn't about actors. It's about character.
Die Hard gave us John McClane along with Holly and Little Lucy. Then there's Al Powell, Argyle, Dept. Chief of Police Dwayne T. Robinson, Reporter Richard Thornburg, Ellis, Special Agents Johnson and Johnson (no relation), Mr. Takagi, Karl, Theo the safecracker and Hans Gruber. That's without getting into great bit parts like Walt, the city worker who can cut the power and the Convenience Store Clerk.
Back to the Future gave us Marty, Dc. Brown, George, Lorraine, Biff and I'll even throw in Mr. Strickland, The "Save The Clock Tower" woman and Marvin Berry.
Die Hard wins. And I didn't even mention the SWAT guy who pricks his hand running through the rose bushes and the great Al Leong as candy bar eating Asian bad guy Uli. (Also known for playing Endo, dispenser of pain in
Lethal Weapon.)