[Rataxes]

In un-modded SoA there really aren't any very difficult fights, maybe Firkraag can be a challenge for a solo player on Insane. Twisted Run could pose a threat as well if you resist the urge to simply trap the entire room with thieves and mages.

With the Tactics Mod though, the number increases dramatically. There are lots of fight in that mod that will give anyone serious troubles, but the by far most difficult is the Improved Irenicus Fight.