Mask first -- it HAS to fit, or even snorkeling isn't fun. Fins are necessary -- you really can't push dive gear through the water without them.
After that, if you're diving water cold enough to require significant thermal protection, buy that, because if it doesn't fit, it doesn't work. In very warm water where you can get away with an ill-fitting wetsuit, not so urgent.
Dive computer. You have to understand it, and it eventually becomes your dive log, so having your own is a pretty nice thing.
Regulator and BC are both arguable. BCs need to fit, so if you're an odd size, that might be next; regulators are nicer if they're well adjusted, so that could be a factor.
Small items like cutting devices, lights and signal tubes, and slates or wetnotes, can be picked up here and there.