My personally thought is:
Russian ZOD(especially when they replaced him silver age superman origin to some kind of human mutant)
Anti-hero(when you look superman team secret origins/files,you will thought he should be enigmatic and powerful character,but when he actually appear,he reacts like some third rate villain)
Satanus,though he was been portrayed really powerful and enigmatic,but always lack appearances(i mean he really to against superman appearances.mostly he just used his mortal guise and occasionally meat superman,even sometimes help superman,he really threaten superman's story is he trying to steal metropolis itself soul in my memory)