Generally, I rank them in order of original release. However, the best of the sequels is, and remains Conquest... The story of the Ape revolt against their human masters had far more social relevance than the rest of them put together. The only reason I'd put the original "Planet" ahead of it is because of Serling's writing.
Number Two: Conform, Number Six! Conform!
Number Six: I will not be stamped, filed, indexed, briefed, debriefed, or numbered! I am a person.