Out of all of my fish I'd have to say my peacock eels are by far the most hardy fish I own.
A while ago I had an outbreak of whitespot in my tank which devastated alot of my other fish and caused a few casualties. The eels never showed any signs of illness and lived through the regular dosings as if nothing was going on. My oldest eel has taken a couple of flying leaps to the carpet, the longest of which was for at least 2 minutes before I could finally stick his squirming butt back into the tank. None of them are any worse for wear and are as healthy, fat and content as one could hope.
Out of all my eels, the only casualties I've ever had were from leaps out of the tank. My upside down catfish would be a close second though, they are another tough one ime.