I always find it intriguing to see the fish looking back at you, their eyes moving to keep you in sight. While goldfish may do some antics, they just look plain boring to me. (sorry)
Triggers act like dogs. Their tails seem to wag faster when they see me.
Sea horses act like cats, chasing my hands around the tank until they can curl their tails around one of my fingers. They leave their hitches whenever they see people around and start going back and forth at the front of the tank, eyes moving to keep us in sight.
Coral beauties are purposefully annoying. They go around bugging the other fish, picking at everything, trying to provoke tangs to chase them, trying to eat the clownfish's poop before it's done "going", just obnoxious.
Diamond head sand sifting gobies are amazing with what they do with sand! Mine always makes a "door" every night at the same time in front of his burrow with sand, and then dives through it into his burrow for the night, the sand falling on itself to re-seal the "door".