I also rather liked Trek to Madworld and Death's Angel.The high points are Planet of Judgment and The Galactic Whirlpool.
On the other hand, the two Marshak/Culbreath "Phoenix" novels were almost impossible to follow, and not really worth trying. And then, there were the "Planet of Judgment-wannabes" (e.g., The Starless World) that just rehashed the "Kirk & co. get roughed up by some superbeing(s)" plot ad nauseum.