Anchorhead:

I recommend watching Aliens once. The theatrical cut. Part of why it's good is that it doesn't try to be like Alien - it is its own movie. If you like it, watch the extended cut, too, because there are good things added.

Definitely skip the other sequels, though.

I agree with you about Back to the Future. The sequels are unnecessary, and not nearly as good as the first. I think it's the whole "chicken" thing that really turns me off of Part II and Part III.

There's nothing redeeming about any of the Jaws sequels, and I haven't watched them in about ten years.

And Conquest was the only Apes sequel I enjoyed.