Spares for your most important things.
Carefully look at all of the equipment in your tank and ask the question "if this single device stops working will my tank crash". If the answer is yes, get a spare. Examples are return pumps, heaters, some form of lighting if you are using a single light.
For dosing, I am a fan of the BRS stuff. It's very economical. You won't need this until you have a ton of coralline algae and/or lots of LPS and SPS coral.
A quarantine tank with Copper Power, Prazipro, heater, airstream and pump are essential to keep parasites put of your display and for emergency treatment of fish. Any other medications I get as needed and don't keep on hand.
Always keep enough salt on hand to do at least two 100% water changes. Always keep enough RODI water on hand to do at least one 50% water change.
Food: keep a variety of frozen, pellets, nori sheets, and flake. You never know when you are going to get a picky fish that doesn't want to eat and need to try another food type.