how do you decide which episode to watch?
Random impulse. Why, today I felt like maybe "Elaan of Troyius", an episode I've seldom watched (compared to... you know... others) but then changed my mind and went for an old standby, "The Enterprise Incident". Clearly I had season three on my mind for some reason.
I guess system-wise it's all about what you're looking for. Want a comedy, an action story, a morality parable, some space romans, a Vulcan with a missing brain... whatever whim takes you.