"Where No Man Has Gone Before," TOS. My 10-year-old son has insisted on an original series marathon while the wife and daughter are at an out of town Irish dance feis. I can't say I'm disappointed in his taste. Son's order of ST series favorites:
1. TOS/TAS
2. DS9
3. ENT - "would be a tie with DS9 for #2, but they swear too much!"
4. TNG
5. VOY - "I hate The Borg!"