It would be very difficult for me to think of an episode of the origial series that I really did not like. The Trouble With Tribbles was a tad silly for my tastes, but even then I have nostalgia for it, and re-watching it was still fun.

My favorites from the original series would be:

The Menagerie
Journey to Babel
Amok Time
Who Mourns For Adonais?
Arena
The Apple
A Taste of Armageddon


As for everything that came after the original series, I consider it all crap. About the only thing I ever liked from the subsequent series were the first two stories with the Borg.