For a long time, Back to the Future, Part II was my favorite movie of the trilogy. It was only with time and multiple viewings that I realized how bad it is, and how good the first one was--even Part III was more internally consistent. Here's why:
[NOTE: I'm assuming everyone in here has seen the movie. If you haven't, and wish to, read no further.]
When the Biff of 2015 steals the time machine, not only does he figure out how to use it with apparent ease, but he comes back to 2015 without a hitch (well, until he presumably dies after getting out). Yet, when Marty and the Doc go back to the "altered" 1985, and Marty suggests going back to 2015 to stop Biff from getting the book, Doc tells him that they can only go forward to the future of the alternate 1985. If so, Biff should have been *equally* unable to return to Marty and Doc's "good" 2015; indeed, they would have never gotten the time machine back at all.
Just some before-work nitpicking.