Copperband Butterfly for me.
Longest fish I had was a Green Spotted Puffer for almost 9 years. I also had a few Archer fishes for 5 years.
Longest actual saltwater fish would be a Stars and Stripes Puffer for 3 years. Kind of sad that for saltwater fish, the longest is 3 years
He is still alive and well though with a friend in Florida. So he is about 6 or 7 years old now.
I would avoid many, many fish personally (mostly the usual suspects for hard to keep fish). Just not worth the effort or risk to keep them alive for. Many are better left in the ocean.