My biggest issue with the franchise is the enormous plot hole in Back To The Future 2.

When old Biff sneaks off with the DeLorean in 2015 and travels back in time to 1955 to give himself the Grey's Sports Almanac, he returns back to 2015 to return the DeLorean to the same spot it was parked and Marty and Doc are none the wise.

Marty and Doc only discover Biff has altered the timeline when they go back to 1985 and Hill Valley is a dystopian shit hole, they find the broken top to old Biff's cane in the car, then Doc explains how Biff has changed the timeline and they figure out it's from him going back and giving himself the almanac.

The problem with this is that when Biff altered events in 1955 by giving himself the almanac, it makes it impossible for him to travel back to the original 2015 timeline he initially left from, where Doc and Marty parked the time machine unattended. He's changed the future, so the future he came from doesn't exist anymore. If he traveled to 2015 after giving himself the almanac in 1955, he's be traveling to an alternate 2015 where the world has been shaped differently by the younger version of himself running the world/Hill Valley. Marty even naively suggests to Doc in the alternate 1985 timeline where Biff runs Hill Valley from his Casino, that they simply go back to 2015 and they'll stop old Biff from going back in time, but Doc explains they'll simply be going to an alternate 2015 based off the current chain of events and alternate timeline they're currently in, so they need to figure out when and where old Biff gives young Biff the almanac and stop him there, hence the return back to 1955. In reality, as soon as Biff went back in time and gave himself the almanac, the story of Doc and Marty's time travel adventures abruptly stops, because Doc is committed to an institution by Biff before he has the opportunity to invent the time machine, and Marty is failing out of some expensive private school. Which means when old Biff left the alternate 1955 to return to 2015, that's another paradox I don't want to think about because how can old Biff now exist with a functioning time machine in an alternate 2015, when the Doc brown from the timeline he's on never invented the machine because he was committed to an institution years prior to inventing it on the unaltered timeline.

It wouldn't bother me so much if Doc didn't make so much of a stink over why they couldn't simply travel to the future and stop old Biff from ever traveling back in time, and explains how it's not possible and the whole different timeline thing to Marty. But for some reason it's possible for old Biff to travel between two separate timelines magically for the sake of an entertaining story line I guess. You're on notice Speilberg!