In threads like these I like to see if anyone named my favorite. I'm sorry to see that in six pages, no one has.
So I now name the true greatest single moment in TOS. It's the scene in "The Cage" where Pike is imprisoned with Number One, Yeoman Colt and Vina and he manages to overpower a Talosian. After he tests a phaser on the prison wall, he brings it over to the Talosian and says:
"I'm willing to bet you're too intelligent to kill for no reason." Points weapon in talosian's face. "On the other hand, I've got
a reason. I think I blasted a hole in that wall. Want me to test my theory on your head?"
And the Talosian reveals the hole in the wall, proving that Pike would have been the equal of Kirk in awesomeness if it hadn't been for that stupid training accident (And stupid Jeffrey Hunter turning down the stupid series and then dying!).