IMO:
I'm going to say mechatau just because he's so random and can actually chain pretty much anyone to death at his level if you get super unlucky. Random is random.
Skaza is a stationary boss that you learn how to deal with over time, and he becomes really easy. Dirigie is also mostly cheesable with a status resist set (unfortunately?).
Pre-nerf 64 heroic takes the cake out of all of them though. I will not forget thee ;p.