I think the first six episodes or so, back when Dr. Smith was a bloodthirsty saboteur, and they were re-using the footage from the original "No Where To Hide" pilot, are some of the best science fiction TV ever created.
Sure. They could have worked a bit harder on the science aspect, but the show (back in it's early B&W days) has some wonderful mood. The music, by Johnny (John) Williams, still thrills me down to my toes!
Too bad it slipped into being outright kiddie fare shortly thereafter. I think there are only 2 episodes that I really cared for, after it went to color... that being the "Antimatter Man" and "Visit to a Hostile Planet".
Number Two: Conform, Number Six! Conform!
Number Six: I will not be stamped, filed, indexed, briefed, debriefed, or numbered! I am a person.