I need to list six:
Wannamoisett, as the pinnacle of Ross's genius in routing.
Seminole, Pinehurst No. 2, and French Lick Springs, as examples to illustrate (1) just how varied Ross's excellent courses are and (2) how everything always comes back to the greens, even if the greens don't follow a single consistent style.
Essex County, both because of Ross's deep ties to the place and as an example of his impact on Massachusetts golf.
Wilmington Golf Course, or another muni that still resembles the original Ross design, to capture that the bulk of his work was designing courses that did not aim to be extraordinary but that are excellent for what they are.
It's amazing to me how different those six courses are, and therefore how hard it is to remove one from the list and still to say that they would give someone a comprehensive understanding of Ross's work. If pushed, I suppose I would remove Seminole.