I think Back to the Future III is a good one. II was never really that good, but i liked the whole tangent universe. As a concept it was cool in execution not so good. The whole revisiting the first movie within the second never really worked.I still like it though but as sequels go its kind of weak.
So many movie series where all i really need is the first film. Back to the Future is one of them, Terminator and Alien are others, and then there is Raiders of the Lost Ark and Jaws, and Jurassic Park. Also the Original Predator. And none of its sequels.