The stuff that it's difficult to impossible to stock up on. Medications. There's some of them that you can't legally stock up on. You can't just have a six month supply of things like adderal, opiates, benzodiazepines, and that kind of stuff. Then there's meds that can't be kept in home and stay at full potency for very long. That's pretty much any meds for autoimmune disorders.
Like, my wife takes three meds that can't be kept in storage without being useless at some point. They get shipped straight from the manufacturer and have to be kept refrigerated during shipping.
Water and shelf stable foods are easy to keep stocked up, and so are water purification items.
After meds, it's ammo. Finite resource that requires a lot of space and tools to replenish by reloading. You can stock up on ammo, and it'll store very well. But it's expensive to do so, and storage space for it isn't exactly infinite.
We're up next to the mountains, and we get (or used to, the last few years have been milder) iced or snowed in a few times a year, so we're used to keeping staples in stock and replacing them as they're used. And it's easy enough to rotate out water in storage for fresh by using the stored water for things other than cooking and drinking. Besides, there's only so much water you can store realistically, so purification methods are a higher priority if you don't have the kind of income/wealth to really bunker up.
We keep a week's worth of water for the four of us. Assuming we get rain during whatever it is, it's realistic to be able to hold out for much longer than that with the bulk staples we keep. Beans, rice, that kind of thing.
But meds? There's only so much you can do about those.