I daresay most of TOS's composers who contributed to season 3 did their best work there, or at least my personal favorites: "Paradise Syndrome" for Fried, "Spock's Brain" for Steiner, "Spectre of the Gun" for Fielding, "The Empath" for Duning. The one exception is Courage; my favorite of his scores is "The Naked Time." But his two season 3 scores have some really good stuff in them. (I love the underscore to Scotty's defiant speech to the Romulans in "The Enterprise Incident." And there's a really great, poignant rendering of "Captain's Theme" at the end of one of the acts of "Plato's Stepchildren," when Kirk and Spock are being humiliated by the Platonians.)
And yet we have never had a release of any of the original soundtrack recordings from the third season. We've had new versions of excerpts from a few -- "The Empath" from Sarabande/Steiner, "Spectre," "Paradise," and "Is There in Truth No Beauty?" from Label X/Bremner -- but nothing more. So the Season 3 set is maybe the grand prize of the whole collection.