What was the funniest moment?

So many.

"1.21 gigawatts!"

The way George walks into the cafe after Marty encourages him to ask Lorraine out. It looks as though he's shit his pants. And then "Give me a milk....chocolate!".

George perving. He's on the tree branch with binoculars and he's still trying to get closer.

"Shit!" - crashes into manure truck.

The Peabody's reaction to Marty getting out of the Delorean in the barn, and the way Marty falls back into the barn after the bullet misses him.

"Chuck, it's Marvin. Your cousin Marvin Berry! You know that new sound you're looking for? Well listen to this!".

George punching Biff and the way he spins around and his expression.

The terrorist getting upset that his gun's jammed.

"That's Strickland. Jesus, didn't that guy ever have hair?".

Doc's facial expressions throughout the clock tower sequence.