I already mentioned Hawkfrost in the favourite characters thread, so I guess that kind of answers the question for me. I just really liked his attitude and personality, and although he was mostly a catalyst for a large internal conflict for Brableclaw, that conflict and their interactions were one of my favourite parts of the latter half of the second series.
I also really like Sol. Just the idea of a manipulative villain who uses tricks and charisma to make people follow him rather than dirtying his own paws was really interesting. I remember the way he was described was very attention grabbing, and I like how the illusion of his power just came crashing down as it became apparent that he was just some lazy asshole who wanted to have people do things for him.
Hawkfrost was similarly manipulative, but whereas he was trying to just raise through the ranks in a somewhat more honest manner, Sol tried to craft some kind of god-like image around himself.
I also find Darkstripe one of the more interesting antagonists, if only because he's not the typical confident manipulative villain. obviously his morals are pretty screwed up, and I've never liked him much, but it is kind of nice to see an antagonist who's not the powerful ambitious ringleader but just kind of following a terribly misguided lead.
I really like Darkstripe as a character too, but it feels weird to consider him a villain. He just seems too… pathetic, and dependent on Tigerstar to be a threat on his own.
Honestly, when I think “What could they have done to make me actually like Ashfur?”, Darkstripe immediately comes to mind. He's the exact same type of character, just better.