I thought the intertwining of the second BTTF with the first, scenes where Marty saw himself in the last film, was clever and I like Old Biff and the "alternate 1985" but I kind of like the original as this stand alone piece.
What I don't like is how Fox was playing himself as Marty's relatives, that was a stupid gag in my opinion. The 3rd one is alright but then you have the same crap again, actors portraying multiple characters.
I would have been perfectly fine with just the original. It said everything that needed to be said.