I always seemed to enjoy the "monster"-ish episodes the most...MAN TRAP with the salt vampire, ARENA and the Gorn, always loved the rock guy from SAVAGE CURTAIN...though I wouldn't necessarily call them "best."

My faves:
SPACE SEED
DAY OF THE DOVE
LET THAT BE YOUR LAST BATTLEFIELD

As for NEXT GENERATION, I love the episode focused on Dr. Crusher where everyone around her seems to be vanishing and no one recalls the missing people...REMEMBER ME I think it was called? UNIFICATION with Spock...I remember that was a big deal at the time...CHAIN OF COMMAND with David Warner as a sadistic Cardassian...