Underwear (Pettis, Bloomers, and Camisoles)
I don't know if this counts as one piece, but whatever. Petticoats are an absolute must for obvious reasons, they are the staple of all outfits! Specifically a Classical Puppets petticoat in your preferred dress shape would be the best, because classical puppets is amazing. Camisoles are good for when you're wearing a less-than-opaque blouse and don't want everyone seeing your bra. They're amazing for modesty, and would be good in black or white depending on your blouse. Bloomers are also mostly worn for modesty reasons, and are yet another good thing to have. They're also quite warm! They help to make sure you aren't flashing everyone.
A blouse in your base/wardrobe theme color
Because, as we all know, you need your blouse to match your JSKs and skirts for it to look good! For sweet and shiro, you should have a white blouse, for classic, I'm assuming off-white would be a nice option, and for kuro and gothic, you would have black as your base- and blouse- color. These base colors go for shoes and purses, too!
Comfortable Shoes in your base color!
These shoes will match everything. The same base colors as above will apply, as well as pink and sax for sweet. These shoes should also be nice and comfortable. Apparently tea party shoes are great for this, so I think all styles besides punk need a pair!
A "Special Piece"
Generally should be something special, limited edition, or expensive. If you're mostly offbrand, your "special piece" could be a Angelic Pretty JSK like this one (Melty Royal Chocolate), or a limited edition OP from an expensive shop.
An Easy-To-Coordinate JSK
Something in all one color, either in your base color or in one you use a lot. This could be your one for everyday coords, a meetup that you haven't prepared for, etc. and is an easy way out, basically. You should have at least one and preferably two of these.