Funny, I made the same mistake confusing "One of Our Planets is Missing" with "Once Upon a Planet". OoOPiM wins that one, largely because of the life support belts.
In the actual
contest "Beyond the Farthest Star" without question. Although my choice is almost entirely because of the first half. It is really the awe and majesty that TOS always wanted to do but didn't have the budget for. Then we get to the second half and it feels like an episode that TOS could have done better. An odd mix.
The life support belts just seem like an accident waiting to happen. They kind of work in the beginning, but are (to me) a distracting plot device in the end. I've thought about what this episode would look like if it were filmed with environmental suits but it's pretty tough to make the Scotty scene work with a suit. Armor I guess?
But it's still one of the four or five best episodes that TAS did.