Loathe to say these are 'the worst', but episodes only ever had interest in seeing once:
The Corbomite Manuever
The Man Trap
Charlie X
Miri
Shore Leave
The Conscience Of The King
Arena
Court Martial
Devil In The Dark
Altenative Factor
The Apple
Fridays Child
Wolf In The Fold
And The Children Shall Lead
Requiem For Methuselah
The Savage Curtain
A lot of the popular 'bad' episodes I find quite entertaining, unlike many of above.