I'm traveling tomorrow and decided on The Twilight Zone to help pass the time. I've never watched it, but here are the episodes already recommended to me that I already have downloaded. Feel free to make your case for any I missed...
  1. Escape Clause
  2. Time Enough at Last
  3. The Hitchhiker
  4. The Monsters are Due on Maple Street
  5. The After Hours
  6. Eye of the Beholder
  7. Will the Real Martian Please Stand Up
  8. The Obsolete Man
  9. The Midnight Sun
  10. I Sing the Body Electric
  11. The Masks
  12. Living Doll
    Suggested by @rulesofjinx
  13. Nightmare at 20,000 Feet
    This and "Nick Of Time" both have William Shatner in early roles and are considered classic episodes. You have a pretty great list already!
    Suggested by @ZacksSoUgly
  14. Five Character in Search of an Exit
  15. The Trouble with Templeton
  16. The Changing of the Guard
  17. I Shot an Arrow into the Air
  18. The Invaders
  19. The Dummy
  20. A Matter of Minutes
    I've seen all episodes and this is in my top 10. One bonus about this episode: it explains how you put something down and can't find it a minute later, and then it's back, again.
    Suggested by @duchess
  21. Examination Day
    This is a dystopian future episode in the line of 1984, etc. Great episode.
    Suggested by @duchess