That’s a tough one! Off the top of my head, I’d say the following:
TOS: The Doomsday Machine
TNG: The Offspring
DS9: Duet - Probably my favorite episode of any ST series.
LD: First First Contact
SNW: Memento Mori - I’ve only seen S1, hoping to watch S2 soon.
I’ve not watched much of TAS, VOY, ENT and PRO, so I didn’t mark them down.
DIS: ??? - I’ve seen up to S3 so far, but no particular episode that really sticks out unfortunately. Probably something in S2.
PIC: ??? - I’ve only watched S1, unfortunately no favorites.
I’ve got a lot of favorites, but these are the ones I probably most enjoy. It can change depending on the day though! There’s been a lot of really good episodes through the years.