My regal anglefish.
He is such a workhorse as he constantly picks at my weedy SPS corals. The result is very little polyp extension during the day and likely slowing down the growth of my SPS. Without him my tank would probably be completely overgrown with SPS. Who wants that?!
All kidding aside (actually I wasn't really kidding), I would have to say my entire army of inverts truly help keep my reef ticking along. From my hermit crabs, my sand sifting starfish, sea urchins, crabs, to various shrimp (peppermint, fire, skunk cleaner, etc). You hardly see them do much but they are hard at work constantly performing chores. Cleaning rocks, cleaning the sand bed, cleaning corals, cleaning fish. Cleaning, cleaning, cleaning! They are my All-Star team. Always cleaning and doing something useful!
Go team go!