Minecraft wiki entry for Diamonds, associated text: Silk Touch pickaxe, with Unbreaking III and optionally Efficiency. This one can be used to collect ores, and if it's diamond, for general mining (the player collects stone instead of cobblestone).
A Fortune III pickaxe with Unbreaking III and optionally Efficiency. This one should also have Mending if possible. If players save this one for mining ores, even an iron pickaxe with Mending basically lasts forever. And if a player uses a Silk Touch pick for general mining, they can keep the Fortune pick safe at the player's base, used only for mining the ores they bring home.
Optionally, a diamond pickaxe with Efficiency IV or V, Unbreaking III, and perhaps Mending, for general mining and as a back-up.