I'm still working on watching and rewatching all the episodes of TOS and TNG, so I don't have a favorite yet, but I can say that I haven't yet seen an episode that I
dislike. Sometimes while I'm watching them, I go through an in-depth analysis (because I find a lot of times that I kinda drift in and out when watching TV/movies and I tend to forget a lot of what happened on screen) and the way that the episodes are planned out and written are, at least most of the time, very nice and smooth.
I did watch the TNG episode "Contagion" recently, and I liked it a lot because they talked a bit about the misunderstood history of the Iconians, and how they were likely bombarded because they were feared. This prompted me to read up online about where the (non)canon books go into more depth on this topic. That gave me a great insight into the lore that I didn't know about.
But ANYWAYS I'm starting to sound like a nerd.

I'm planning on watching more of those 2 shows, and then once I'm finished I'll get into DS9, the Voyager, etc.