Frankly, my two hitchiker gobies. Personality times a thousand, reef safe, peaceful, and max size about an inch and a half. Also managed to take home a porcelain crab - the filter feeding one.
Best are the zoanthids, they're so varied and abundant. And if they can take the abuse of a tide pool onva summer's day, complete with tourists walking and peeing on them, they're hard to kill.
I've had luck with some of the local anemones, also damn near impossible to collect without hurting them. Found a few on things that I can legally take, which is a fist pumping moment.
Also have had luck with micro starfish, cowries, and some nudibranch hitchikers - still growing five months after collection.
Only real failure was the giant chiton. Kept exiting the tank and waiting for the tide. One time I didn't find the poor guy till it was too late. :-(