Being relatively new to this hobby... the most interesting critters for me have been my queen conch (who died this summer, I am still trying to figure out the cause of that), a lawnmower blenny that loved Nirvana, and my clownfish.
I could spend hours watching the queen conch jump across the sand, watching her eat, watching her bury herself into the sand bed and hide for a month with just a single spire of her shell poking out. This is quite possibly the neatest animal I have kept so far. In the future, I'd like to build a biotopic system around these animals, but that's been put into the pipe dream box for now.
My lawnmower blenny never took prepared foods. He was a neat fish though, probably the most awesome personality I have had in a fish so far. Every time I put on Nirvana in the tank room, he'd come out and kind of dance around in the water. It was neat. I watched him starve to death, and I cried when I realized what was happening.
My clownfish... I love my clownfish. This probably seems weird, because they're such a common fish in the hobby, but I just think ocellaris are neat. I love to watch them swim, I love to watch them pillow in their colt. Heck, I just love to watch them. Now that they are spawning, they've got even more fascination for me. I'm always checking around the colt for a new patch of eggs.
J.