Grouper, sharks, big morays, many puffers, Mocha clown, the list goes on. Just about anything super predatory or aggressive counts. Of course, in the right size tank and with the right tankmates, there are almost no fish that cannot be kept with something else.
Perhaps the only fish, fresh or salt, that comes to mind is the electric eel, any fish with it would soon die or be in a perpetual state of torture from the constant shocks.