The Doomsday Machine
It seems strange that a presumably scientifically minded starship captain would look at what is obviously a ship and start talking about the devil. Although the maw of the ship does look a lot like the Eye of Sauron...
So if the Doomsday Machine needs to feed on planets to fuel itself, how did it cross from one galaxy to another?
You're out of line, too!... Sir. - Great timing
Shouldn't McCoy have the authority to order Decker to submit to examination if he suspects he's been compromised? And to remove him from command if he refuses to submit to the exam?
When targeting a seemingly indestructible ship, I think I'd at least aim for the obvious 'seams' in its outer hull. Better chance of finding a chink in the armor than just punching straight through.
An episode like this really makes you roll your eyes extra hard at the lack of seatbelts. Not only does Sulu fly halfway across the bridge from a single hit, but Kirk and Scotty can't even keep their seats while accelerating at a speed explicitly described as 'just enough to get us moving, no more than that'.
It's interesting how closely this parallels ST09, right down to Spock debating the commander of the ship about fighting vs regrouping. Only, Kirk going in in complete defiance of logic makes him the hero in 09, while here it makes Decker a tragic figure just barely spared from becoming a villain.
Overall, I'd say a very solid episode. Decker was too over the top at times, but he had a truly heartbreaking story with an incredibly appropriate end. Far more nuanced than I actually remembered it being. And certainly a nice tough that his sacrifice did ultimately lead to the salvation of Rigel, after all, even though it logically couldn't possibly do so.