My favorite "go-to" button is the shell button that looks like mother of pearl. Local stores are usually low on the size I need on the day I go shopping, so I think I'm going to pursue buying three sizes in bulk for shirts, blouses, and jackets. I believe that online tailoring supply stores carry them by the pound.
I have a large basket of buttons on the cards, but I never use them, just add more when they're on sale.