Favorite fish I haven't kept would be flagtails. They're adorable and always seem happy to interact with people. That and their adorable little smiles, so cute!!!
Favorite species I've owned would be goldfish. Never shy, amazing bright colours, also always happy to interact with you whether in a pond or tank.
Favorite individual fish I've owned was my big geophagus brasiliensis, he was amazing in a way that words can't describe. R.I.P big man.
On a side note I've always loved the way knife fish move. In the past I kept 2 BGK's (on separate occasions) and both of them would have me mesmerized every time I seen them out and about. Very elegant fish.