My "must have" list is more sparse. Most calibers fall onto my "sure, it would be nice to own, but not necessary" list.
1) A middle of the road handgun caliber (9mm, .38 Special, 40 S&W, .45 ACP). Debates between these calibers are silly, imo. They all do the same thing, with pretty much the same terminal ballistics. Pick whichever one you're most accurate with, and can afford to practice regularly with - that's far more important than the actual caliber.
2) 12 ga. Extremely versatile and hardly needs explaining.
3) A magnum caliber in both a handgun and a lever action. The .357 Mag remains most popular in this role for a reason, but .44 Mag is a great choice as well, and also offers the dual-caliber capability.
4) A versatile bolt action hunting caliber. My choice these days is .308 - easy to find, comparatively inexpensive and available in a wide array of loadings for just about everything. Even during the pandemic, when a lot of other options were hard to find, .308 could be found pretty easily. There's a lesson there.
5) An AR chambered in 5.56 Nato. Because.
6) .22LR for all the reasons that have been abundantly stated here and elsewhere. Everyone should own one. Or several.
That's really all I truly need or would put on a "must have" list. The rest are just icing on the cake and something else to spend money on.