I think this is just really personal, if everyone's collections were the same, we wouldn't need the Bag Showcase thread! I do think it's nice to have a wide variety of styles/colors/brands, but others prefer to just collect many styles in one brand.
I don't think there's anything you must have in your collection, because not everyone's going to like everything. For me, I would never buy a spy bag, that's just not my taste, and I wouldn't want someone telling me my purse collection was incomplete without it.
IMO, the idea that you must have certain bags, I assume you mean certain brands as well, doesn't really appeal to me, as it smacks of being brand conscious and following popular consensus of IT bags and all.
I learned this the expensive way, when I was much younger, I bought quite a few LV bags as this was THE brand to carry where I'm at, but now they're just sitting in my wardrobe. After collecting bags for a while, I realise I have a strong fetish for bags with great slouchy soft leather, which I use all the time instead. Now I buy bags with great leather texture across brands/price ranges. those bags now make up my essential collection.
To answer your question , certain handbags which may be collectible may be Hermes Birkin and Chanel 2.55 which are deemed timeless/classic, but you would need to maintain them in great condition.