Couple of them
 

Love my coral beauty angelfish, basically the mod in the tank, swims around and nobody screws with her but if others are fighting/getting abused she will just swim by and it will stop. Pretty funny to see actually.

Copperband I bought just because it was eating everything, and it turns out when I got it home it also liked aptasia, so that was a plus.

And of course the Lawnmower blenny, one of the most inexpensive fish but looks pretty cool and does an amazing job of keeping all the algaes in the tank in check.

Doliatus rabbitfish. There's not a species of algae it won't eat. Even picks at dinos. The only place I have algae growing is where corals have grown in to tight for it to reach.

Second to that is my Copper Band butterfly. I cleared out the aiptasia problem in my tank with berghia, and now I'm pretty sure my Copper Band is responsible for keeping them out as a few have popped back up in the sump, but not a single one in the display.

Finally, and probably most important of all - my long horn cowfish, because it's so freaking charismatic even my partner Kelly, who sees my tank as nothing more than a giant money devouring vortex, loves it. The amount of leeway I've been given on the tank and the relationship/tank capital I've banked by having that guy floating at the glass begging all our dinner party guests for food while Kelly talks about how cool he is is quite literally worth that fishes weight in gold.

Once the bully ever since he was acquired, my Humbug damsel had shown a very unique turn about. He would be a defender of bullying that he was once champion to protect newly introduced fish. I first observed this with a Clarkii he saved from being torn to shreds by a spawning percula pair. He would literally get in the way of an attack and stay on his guard in the corner where the Clarkii found refuge. It happened 4 other tines with a purple Firefish, another clarkii, and a yellow tang he protected from a much bigger purple tang.
But alas, having moved him and all my fish into the bigger 180, he's reverted back to being a bully and harasses anyone new.