While there are some later Treks that had some great drama, you just can't beat Shatner and Nimoy playing it straight and serious. Kirk in the Corbomite Maneuver or in the Man Trap, for example.
I don't know how it holds up on repeat viewings because I've only seen it once (last year or the year before), but with "The Corbomite Manuever", I was shocked by how effectively suspenseful it was.
That was the first time I was really, really excited by a dangerous situation on the show...unable to imagine how the Enterprise would escape certain doom. I was so impressed by how original and dramatically satisfying the solution was. I guess I'm afraid to watch it again because I found that whole puppet thing in the final reveal a bit hokey, but it was a real nailbiter the first time around.
