Dude I've been watching Back to the Future 1 and 2 like every night for the past month! I think Back to the Future is like my all time favorite movie.
Anyway, I think Back to the Future Part 2 is about as good and perfect as a sequel can get and when you take into consideration of how the first one ended, the two Bobs did an incredible job writing a sequel for that, with Jennifer being in the DeLorean and all. Biff Tannen was an unbelievabley good character in the sequel, which made it a big highlite of the movie for me as well as seeing the future and Marty having to go back to 1955 again while avoiding himself. Awesome, awesome stuff and I think BttF2 has the most amazing special effects than any movie. I still slightly prefer the first one though since it's pure classic chram, but BttF2 was the perfect sequel.
It's a lot like Star Wars to me where the first one is a timeless fun adventure, the second one being an epic sequel, and the third being duller than the others, though I think BttF3 is much worse of a sequel than Jedi was. Strangely enough, my favorite parts of BttF3 and Jedi where the beginning and end with the middle being boring and silly.