Really, anything large and responsive to people will fit this bill.
When it comes to impressing non-fishkeepers, I think the arowana takes the cake since it is unlike any fish that these people have seen before.
When it comes to impressing fishkeepers, especially us MFKers, it gets much more difficult to narrow down. But I would say that the following all fit the bill:
GATF
Armatus
Large black rhoms
Fahaka puffers
Umbie
Dovii
Large ST dats
Giant endli and congicus bichirs
Large pbass
This list could be never ending. But for me, the most impressive fish is a large ST. When I watch JohnPTCs webcam, the first fish that catches my eye is his large ST that is always hanging around the front windows.