There are only two that I'd call unwatchable:
That Which Survives
- It just strikes the wrong tone throughout, especially with Spock's exessive sarcasm. Feels as if the writer had never even seen Star Trek.
- I can truly say I have never made it though this episode without actually, literally falling asleep. Maybe I shouldn't judge it without having seen all of it, but I don't think that's going to be a realistic possibility.
But I'll add a third:
- ... although it's so mind-manglingly bad that I find it immensely entertaining.