Not really a main antagonist, but, I kinda like Orson as a villain.
The moments where his wife speaks off-screen, and his general attitude during the chapter where you fight against him, is really twisted and creepy, but, he is a tragic character.
Also, he isn't corrupted by lust for power, or only by some dark magic spirit god voodoo thing. He becomes deranged because of his intense love for his wife, and, he is a renowned knight, which means that his betrayal doesn't necessarily mean lack of loyalty to Renais, but only that it didn't come close to what he felt for Monika. Or was it Monica. Eh, forgot.
He has some similarities with Hardin, now that I think about it.