I agree with the fahaka being one amazing looking fish, but with a very bad temperament if it doesn't like its tank mates (not that its ever advised to have any other fish in with a puffer)
One that surprised me the most were some panda tetras (Aphyocharax paraguayensis) I tried to keep. They say exodons are "mini piranha", well panda tetras are mini exodons (but probably meaner). I put them by themselves first to see how them would behave (I've heard of them teaming up on larger fish). I started out with 7. In like 3 days, I had one left and he was beat up pretty bad before he killed off everyone else