Hardest fish for me to keep alive has been Clownfish... True Perculas, all of them.. I have basically decided that I won't keep clowns anymore, I've lost 5 of them in my time.. I don't know what it is about them or me, but they just do not survive more than a couple weeks.
I can't really comment on my longest lived fish, but all the fish in my current tank have an opportunity to take the title.
