I am in the process of marathonning the entire show. I have watched every half-hour episode and I am in the process of finishing up season 4. Been a fan of the show for years, but never got to sit down in bingewatch it in full.
My favorite episode, and probably one of my all-time favorite pieces of media, is Walking Distance. Every element of the episode works perfectly. The script which holds true a message of enjoying life in the time you lived in instead of wanting more from the past or future to create imbalance. The atmospheric childhood-like presence created not only by the set design, but the wonderful absolutely BEAUTIFUL score by Bernard Hermann, and the sense of sincerity created within the desperation of Martin Sloan trying to talk to his childhood self is so sincere. The acting is great too, especially Martin Sloan's father near the end who appears as sort of shook, but also, a comforting figure who wants the best for his son, but is also weirded out by seeing him as an old man. It's so fucking brilliant, it really is. Really cinematic feel to it, too. They somehow mastered the show in the fifth fucking episode and nothing comes close to beating it. Not even To Serve Man and The Monsters Are Due On Maple Street. A television episode with this kind of pure sincerity couldn't be made today.