It's really hard to limit to just 5 things for a reef tank, and since we are talking saltwater here I'd assume a refractometer is a given, but I'd say a good light, definitely a couple QT tanks with spare filters, heaters, etc. Water test kits, RODI, and I always use vitamin supplements when feeding my fish. A good, varied diet is key to happy, healthy fish in my opinion and I'm a supporter of seachem, absolutely love their products. So whenever I feed I add a capful of vitality and a capful of garlic guard and let the food soak for 5 minutes. Also when I feed it's a combination of seachem flakes, brine, and small bits of seaweed. Occasionally I'll add a few pieces of krill, and I alternate between brine and mysis. I also put 2 strip of seaweed attached to a clip in my tank every other day. If you're looking for 5 pieces of equipment not to go without I'd say protein skimmer, ato, circulation pumps, light, rodi.
To recap:
Good HO light
RODI
Test kits
QT system
Food Supplements