I do think the key to the choir patch, before all else, is to start off with the proper wave shape. In such a case, that requires either a particular digital wave shape, or a substantial number of analog wave forms that can be combined to imitate that wave shape. And then, voice number. This is why I think a twelve-voice poly synth like the Prophet 12 is by no means over the top. But, not having played a Prophet 12, I don't know if it has the right wave shape. I've asked this question at least a couple times on P12 threads, without receiving a clear answer. For now, I'm content to consider the Poly Evolver Keyboard (or Prophet VS) to be among the best means, in the DSI line up, for creating a decent choir patch ().
Listening online to those patches that are called "synthesized choir," I find that they generally have a throaty quality about them, but little more. But many are so covered in a dense digital smog of aliasing, filter sweeping, and white noise, so as to have an ethereal character, that it's not quite accurate to call them choir patches. I would consider this example to be more accurate:
Many moons ago, I created a choir patch on a Juno 60 that was fairly good. It was the Roland filter that made it possible.