Great episodes:
Balance of Terror
Space Seed
The Devil in the Dark (okay, maybe not great but a personal favorite)
Space Seed
City of the Edge of Forever
Mirror, Mirror
A Piece of the Action (see Devil in the Dark)
The Changeling
Terrible episodes:
And the Children Shall Lead
The Spectre of the Gun
The Squire of Gothos
Miri
The Apple
Plato's Stepchildren
Balance of Terror
Space Seed
The Devil in the Dark (okay, maybe not great but a personal favorite)
Space Seed
City of the Edge of Forever
Mirror, Mirror
A Piece of the Action (see Devil in the Dark)
The Changeling
Terrible episodes:
And the Children Shall Lead
The Spectre of the Gun
The Squire of Gothos
Miri
The Apple
Plato's Stepchildren