The one that started a Franchise.
So, here we are. The very first Star trek episode, except it's officially no.2 That's because the 1st Pilot, The Cage, is considered part of the Star Trek universe. But it's not on my DVD box set, but is on Season 3 mysteriously. So it'll get reviewed at the end of the TOS.
Anyway, this is an excellent pilot and does a great job of introducing some of the crew. It's quite strange that Bones, Uhura and Chekov are missing. And Spock has some STRANGE eyebrows going on, as well as being quite emotional and shouting all the time, even when performing the simplest tasks.
The plot is one that will crop up many times in the future. The Enterprise encounters a strange force that elevates one of the crew (Gary Mitchell) to Godhood Absolute power corrupts etc. and Kirk ends up having to fist fight him whilst potential god no.2 (Liz Dehner) fights the changes.
Having Gary be one of Kirks best friends makes it a lot more problematic when it comes to finally having to stop him, but he maybe turns evil too fast. Liz takes a lot longer and is more tortured by the fact she is leaving her humanity behind. It's entertaining and only the rather straight forward A to B nature of the plot and familiarity of the Human with god like powers brings it down a notch.
Spock arguing with Kirk is good stuff, but misses that McCoy dynamic. But considering it's the first proper Trek made it's amazing stuff and stands the test of time well.