Entertainment

You know, 'In The Pale Moonlight' is always the default answer to this question, but I'm going to give two answers that I never see. These episodes are top for me for very different reasons.

First, 'It's Only A Paper Moon'. The episode where Nog is sinking into depression inside the holodeck after losing his leg in battle. This is the most realistic depiction of PTSD I have ever seen on TV. Usually in media PTSD is represented as literally flashes and hallucinations with the person becoming acutely unhinged. The reality is often a lot more subtle as the person withdraws to somewhere "safe". It takes the rest of the cast such a long time to even notice something is going on with Nog, which kind of feeds into the emtional isolation.

Second, 'House Of Quark'. A totally different tone than above, this episode is just so fun. No other Trek show could have pulled this off because it's about two alien species and cultures with very little interference from Starfleet human centered culture. Everything from the initial bar fight to Quark doing all his math for the Klingon council. It's all brilliant.