TNG:The Ensigns of Command
A true Trekkie can't say there is a best episode. There are definately episodes that had more action than others. And episodes that were more scientific than others. And ones that people didn't want to watch more than once.
New episodes that you never saw and are good tend to become more enjoyable than previous ones. But once you have watched them all, I don't see how you can have a favorite.
Guess I ain't a true trekkie, cause I would say DS9: The Visitor
If I HAD to pick a favorite, it would be First Contact. I found it fascinating to see an alien culture at our (well, a bit ahead of our) level of development.
I like the one where Picard dances around the bridge in a barbershop quartet hat with a cane singing about the alphabet.
Everybody says "Yesterday's Enterprise"--but really it's "Deja Q". The bridge scene with the Mariachi band and the cigars is vintage Trek. He's immortal again--OMNIPOTENT AGAIN!! Don't fret, Riker--his good fortune is your good fortune
DS9:In the Pale Moonlight
"THERE ARE FOUR LIGHTS!!!!"
Tie between Yesterday's Enterprise, Balance of Terror, and The Inner Light.
The one with the photon torpedoes.
All of the Mirror Universe Episodes....Especially the ones with Smiley in them...I dunno why, I love him! But the main reason is the whole idea of a universe where everything is inversed and opposite....It's truly fascinating!