I love the whole BTTF trilogy, but the first is definitely my favorite. There is one glaring plot hole in my mind in the second. According to the time travel rules established in the series, if you go back in time and change the past, the future you return to follows the changes you've made. When old Biff goes back in time to give his younger self the almanac, he then returns to the same future that was properly his without any change to the timeline just yet. He should have returned to his own altered future rather than the future where Marty and Doc had been.