I love, love the 1955 part of BTTF II. On the other hand I don't like the 2015 part (well I like some of it but not overall). The way two timelines come together neatly and how Marty from original timeline has to avoid seeing his first future travelling version in the1955 timeline is brilliantly done. The ending is great too. In fact it's one of my favorite endings to any movie ever!