Hi, I don't think there is such a thing as an ultimate collection target.
Every individual collector is different and what interests him/her about watches is different. Ideally, I think a collection should represent the owner's feeling toward a subject/topic (and not that a collection of things defines the type of collector, be it watches, cars, stamps, etc.).
What is an ultimate, or grail, cannot be defined by others...it can only be defined by your interests and passions.
Cal. 135 is a marvelous, historically significant, movement. You had mentioned (offline) Observatory Chronometers. Please be aware that Chronometre Royale is a VC trademark name. It does not necessarily mean that a CR was an observatory chronometer, though some of them have been sent for observaory trials and passed. An observatory chronometer muse have been individual sent for trials and passed to earn that name. It is much harder to obtain than COSC, which is why they are so special.
Also, Observatory Chronometers may not have all been beatifully finished. They were built and regulated for a specific purpose: to pass Observatory Time Trials.
So I go back to this idea again: an ultimate/grail depends on what you value and want.