Sorry for being late to the game. I definitely agree that Twin Hills and Hillcrest belong on the list. Oakwood is really interesting too since it is both a fine golf course and, to my knowledge, essentially unchanged. I know that Tiger Bernhardt (God rest his soul) praised Bayou DeSiard as a hidden gem but, like Chris, I haven't been there yet.
Maxwell isn't quite the household name as Ross, Tilly and some of the other ODG's. Plus much of his original work isn't in major markets. So the recent restoration craze has largely passed him by. While I'm hoping that Old Town and, to a lesser extent, Hillcrest will change that, sadly, all too many of his courses have already been irreversibly altered. For example, Melrose should be on the "essential" list since it was the course that put Maxwell on the map and was by all accounts an exceptional design. Unfortunately, that course is pretty much long gone.