For me it's the Pike Blenny, one of my favorite fish I ever had. Would pay a pretty penny to get another one if anyone collected them, unfortunately no one does. It's from the tube blenny family.
Picture of the one I had is in my Avatar (OrangeThroat Pike Blenny)
Cool fish lots of personality, would display by puffing out his gills if felt threatened, would live in a cocoworm tunnel or SeaFan tube. Apparently not that rare, just not collected often.