Capra Demon was the one that took me the longest to master, so I'd have to go with that.
As for the second game, it has to be Sir Alonne just because of the area you have to get through before you fight him.
English
-
Sir alonne? I can't remember which boss that is...
-
He's in the second DLC. You have to kill the Fume Knight before you can fight him.