After reading IDW Transformers I started to think... What type of villain for classic hero (just, lawful and overall good with proper moral ideals - somebody like classic paladin) is the hardest to deal? As a whole without cheap powers and etc...
Someone in the morally grey zone, who can do good and evil within a hat's drop, someone who can serves as the Hero's assistant and betray him once their objectives are not aligned. Basically heel face revolving door.
Someone who is morally righteous and committed..just disagree on what is right and wrong...but MC has to respect viewpoint.
Family. Mainly because they know you best and you naturally let them in. And even if you defeat/get away from them, you're left with some scars and/or trust issues. Basically becoming a jaded human being.
How about Villain that is basically result of... Hero's failure or failure of his ideals? Like Megatron from IDW continuity. Would Hero be able to deal with something like this on emotinal level?
Someone with a high position like president, king, etc. Since even if they do evil stuff the citizens who receive protection and prosperity(albeit abysmal) will also colapse. Unless there is a suitable replacement.
Someone who means well but does evil accidentally. Something like a 'pig-like team member'. They aren't your enemy so you can't fight. They aren't doing it on purpose so speeches and trying to convince them do nothing, even if they listen closely and agree. I don't know if a villain like this exists in fiction yet, but I think that kind would be the hardest to deal with.
Someone tenacious like a cockroach, can survive anything you throw at him/her. Your ex. (Hath no fury like an ex scorned) Your mother-in-law. (She always wins) Your dentist(Torture experts). Your doctor.(Patches you up, but you go ahead and get beaten up again. Reserved the biggest needle injection just for you).
A really good tactician that can twist all his evil plot into a morally grey or even justified area and not get caught easily. Like how Tanya twisted the law to shell that city in the manga/anime. Riddler was kinda good but he always cut some slack around the end so its lackluster. Edit: I just remembered Screenslaver from Incredibles 2. She has justified resentment for the heroes but took a quick hit rather than an elaborate fight so kinda lazy.
When a society views the heroes actions as as wrong due to thier set of beliefs. (Ex: marrage at the age of ten. Or slaves.) The villian embodies these traits and are seen by most people as a paragon of society..
If we are talking about a morally just hero, the hardest to deal with would be one that takes advantage of the MCs morals. For example, if hero saves X, Y dies. Aka, a villain that traps the hero in between choosing to fail on his morals no matter what option they pick. PS for anyone who says morally grey villian, that is the easiest to deal with cause just heroes tend to view things in black and white. If they believe the grey is black, then its black.