I'm not sure I'd give scores to the episodes, but I would definitely say my favourite episodes are:
Collision Course
Black Sun
Death's other Dominion
The Infernal Machine
Another Time, Another Place
War Games
But I love them all really. Ring around the Moon, The Last Enemy and The Full Circle are the only ones I find disappointing, but even those have things to enjoy about them. I've not got a problem with the premise, as incredible as it may be, I think probably because the show goes with it - it's big, bold and in-your-face, and they don't try to wrap it up in a mumble of unconvincing technobabble. The Moon's been blasted out of orbit - deal with it! Although of course, interestingly, the series itself questions the likelihood of the event, which leads to the idea of cosmic intelligence. Now, I don't know what cosmic intelligence means, and the series itself doesn't offer any definite conclusions - that's a plus point. If they explained these things, it would just be a mundane, generic sci-fi show. It's about the mysteries of life and the universe - it's about faith and hope (like the number of times Koenig broadcasts to the base his belief that they will survive).
Personally, I think of "cosmic intelligence" in terms of a sort of universal group mind, which is a bit of a Stapledonian idea. (Though Koenig, as a Jew, equates it more literally with an actual individual God - so who knows?) Presumably that's what happens to Arra and her people, they ascend to a new plane and join the cosmic intelligence. One assumes that the ancient Arkadians are part of the group mind too, which is why they use the Moon to bring life back to their original world.