Surprising entries and misses!
The "Go slow…" maxim was the best, and least used, invention/innovation for modern and ancient reefkeeping.

"Don't over stock…" is #2 on both lists.

Tunze Stream pumps showed the right way to flow a tank. Practically speaking, nobody does it any other way now.
Affordable, accurate-enough, precise-enough Ca and alk test kits. It's not 100% clear to me, but I think Salifert led the charge and today most of the available options are pretty serviceable.
Aqua Lifter "dosing pump" - historically nothing like it at the price.
Who didn't make the list?
The skimmer is personally my favorite device, but also one of the greasest disappointments given what we've discovered about their overall lack of effectiveness. (Referring to the skimmate testing ..). This fact puts a pretty hard cap on what I'm willing to spend. Also will consider going skimmerless on future designs.
LED lighting may make the list in another year - I'm hoping for at least one more wave of innovation that'll make it right for everyone. Most people should already be able to make the switch to LED for about the cost of one or two bulb replacement cycles on their current T5 or halide fixture. This, however, requires a fair effort in DIY. Hoping the commercial market continues to look at new tech for implementing LED at more popular price points.