Not an episode but ST IV for "wessels" comes to mind

Koenig said one of his favorite episode was “Spectre of the Gun”.

The Psi Corps is Mother, The Psi Corps is Father.

In Gamesters of Triskelion he's featured rather prominently. Mirror, Mirror is a great one because he gets to really cut loose.

whats scary was he was the young guy. the ensign. several episodes he is infatuated with girls. like the ok corral one.

I opted for "The Tholian web", and it opens on Chekov. Maybe he's not super prominent in it otherwise, but it's a peak TOS episode.

Edit: Watching it now, he has some good away team scenes, and that great freakout on the bridge when Kirk's fate is uncertain. Definitely a Koenig showcase!

The Starlost - The Alien Oro