Personally, I think the two Back to the Future sequels manage to maintain the quality of the first one. But aside from that, I certainly agree that we are very lucky that ESB is as good as it is. That was pretty much the idiom that Lucas had at the time: ESB had to be good in order for Star Wars to survive. If it hadn't been, it would just be Star Wars with a few crappy sequels. Probably no prequels at all.