Another difficult fish: (More "normal" fish will surely follow)
How about an Arowana (Osteoglossum bicirrhosum)?
You don't see too many of these guys in normal aquariums due to the fact that they grow to ~4 feet in length and have a mouth large enough to swallow small birds wich it can jump out of the water to catch!
One thing that makes this such an interesting fish is the way it swims – it slithers almost like a snake, just below the surface.
My favorites:
Terrarium – Snapping Turtle
Marine Aquarium – Lionfish
Freshwater Aquarium – Arowana
Anybody see a pattern here?