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To quote the author, "The Most Important Piece Of Gear. . ."
A wet suit won't do you any good scubadiving if you don't have breathing gas.
A dive buddy won't do you any good scubadiving if you don't have breathing gas (ruling out the premise that you're breathing from his tank).
Your brain . . . I don't really consider that "gear".
A mask won't do you any good scubadiving if you don't have breathing gas.
A light won't do you any good scubadiving if you don't have breathing gas.
A spring heel strap won't do you any good scubadiving if you don't have breathing gas.
Fins won't do you any good scubadiving if you don't have breathing gas.
A BCD (regardless of type) won't do you any good scubadiving if you don't have breathing gas.
A computer won't do you any good scubadiving if you don't have breathing gas.
A safety sausage won't do you any good scubadiving if you don't have breathing gas.
A certification won't do you any good scubadiving if you don't have breathing gas.
A travel bag won't do you any good scubadiving if you don't have breathing gas.
Did I get them all???
Now, having said that the tank is excluded, then THE MOST IMPORTANT piece of gear would be that piece that brought the breathing gas from the tank to your hungry lungs . . . hence . . .
THE REGULATOR
Just my opinion.
the K