1. Spock's Brain (TOS) (who didn't see that coming?)
2. Code Of Honor (TNG)
3. The Last Outpost (TNG)
4. Run Along Home (DS9)
5. The Phage (VOY)
Bonus: 6. These Are The Voyages (ENT) (What had been Enterprise's only
really good season ended in a total shitbox of a series finale.)
Discovery isn't Star Trek, so it doesn't count.