Deep Space Nine.

It had the best ensemble — literally every main character, and even some supporting ones, were fully fleshed out. Contrast to TNG where Beverly Crusher is just there to spout medical jargon, Geordi is just there to spout treknobabble, and so on.

I enjoyed its ability to have more recurring plots than other classic Treks, while also not requiring you to watch the whole thing start to end.

I enjoyed its bravery in tackling all sorts of genres, pivoting from a lighthearted baseball episode to an episode about Nog losing his legs, while still not feeling like a whipsaw because the episodes are largely self-contained.

I liked its exploration of darker themes primarily around the Dominion War, while still being very grounded in a largely optimistic view of humanity and its future.