my Black Aro. Not the coolest fish I have nor my favorite fish... but is just kinda special and nice and you don't see 2ft ones too often, and I think it would be hard to replace.
#2 would be my metallic or common sheatfish... same reason. Not easily replaced.
Odd thought, my most PRIZED fish not necessarily my favorite fish (Favorite is my aba, mbu and humerosa ray)