I like BTTF2 precisely because it explores the pitfalls of time travel, in a darker way than the original.

Conversely, I dislike BTTF3 because it feels too much like a simple period film; aside from the wonderful finale on the train it's really just a comedy Western.

All of which I guess means I just agree with Bingowings.

Oh, and the original idea for the plot of BTTF2 was apparently that Marty goes back in time to Woodstock and prevents his parents from having the sex that led to his conception. Which would really have been derivative of the first film.