What was the funniest moment?

Jesus Christ Doc, you incinerated Einstein.

Marty - "Jesus, George, it was a wonder I was even born."

Doc - "Maybe you were adopted"

Doc - "I'm sure that in 1985, plutonium is available in every corner drugstore, but in 1955, it's a little hard to come by."

Doc - "There's that word again. "Heavy." Why are things so heavy in the future? Is there a problem with the Earth's gravitational pull?"

Lorraine's Dad - "He's an idiot. Comes from upbringing. His parents are probably idiots too. Lorraine, you ever have a kid who acts that way, I'll disown you."

Nobody thinks that when Doc builds the model of the clock tower than his model scorches the wind up car is hilarious??? The car is seen driving away engulfed in flames then crashes into a trash can of oiled rags. The noise Doc makes when it ignites is hilarious

"Are you telling me that this sucker is NUCLEAR?!"

Doc: (referring to his mind reading device after Marty explains that he's from the future): "My God...do you know what this means?"
Marty: "What?"
Doc: "It means that this damn thing doesn't work AT ALL!"

I also love when Marty falls down after Lorraine throws him his pants.

"Do you know what the Preservation Room is for?"
"Delicious jams and jellies?"

There are also lots of non-verbal funny moments that I love.

1. The Doc's face as he looks at Marty when Lorraine follows him to Doc's garage!
If looks could kill. So funny! That gets a big laugh in cinemas too.

2. Also when Marty is in the space suit and Doc puts the plutonium in the car. It gets sucked down quick and Marty jumps back in fear! (subtle visual gag that one).

3. Marty falls over backwards in the barn when he arrives in 1955. Superb slapstick acting from MJF.

4. Marty falls over in his purple underwear!

5. When Doc is demonstrating that the Delorean is a time machine, he revs up the engine, using a remote control. Marty is scared, so he steps further away from Doc. Then Doc gives him a look as if he is really offended that Marty stepped away!

6. When Doc sticks the suction cap on Marty's forehead. This is doubly funny as it's straight after he takes it off his dog (How could the dog tell the Doc if he can correctly mindread?!!! And the dog's fur would not allow suction anyway!)

Plus too many more to mention. My all time fave movie / trilogy.

"Last night, Darth Vader came down from planet Vulcan, and told me that if I didn't take Lorraine out, that he'd melt my brain."

That and

"He's an idiot...comes from upbringing...parents must be idiots too."

"What are you doing here McFly?" I told you not to come here......

"Eh, you are not McFly" -(imagine this with Biff´s face expression and his voice)- "but you look like him"

As an engineer my favorite line is when Doc says “It works! It works! I’ve finally have built something that works!”

I think that scene is supposed to be reminiscent of when Dr. Frankenstein says “It’s alive!”