Depends on my mood. Sometimes I want a Kirk-heavy, very serious episode so I'll go with an early first seasoner. Other times, I want to relax with an episode where everyone is comfortable and friendly, so mid-second season gets a pick. Then there are times I want a dead serious, but over the top episode with spooky overtones. Third season ahoy! Within those parameters, I make more choices:
Space or planet?
Climax with phasers, speeches or a fistfight?
Episode I've seen a million times in three years or one I haven't totally worn out?
Am I in the mood for a less- than-good episode?
Do I want the new effects, the BD or DVDs? Or the old laserdisc prints?
This all sounds complicated, but making these choices at the speed of thought isn't so long. These things make up my moods.
Other times I'll just grab and DVD and pop it in and just hit play.