Moenghus and Kellhus are my first instinct, but I'm not sure Moenghus makes sense.
I think the most objective answer is that Kellhus and Cnaiur are the two characters that go 'most mad'.
Cnaiur and Akka maybe, though I'm not sure Akka really 'goes mad' in the same way as the others.
Who else? Xerius and Conphas end up a bit nutty.
Xinemus definitely goes insane.
Speaking of guys who lose their eyes - Iyokus.
Maybe more esoteric, but The Three Seas entire, as well as Kian, under the tetelage of their respective Dunyain leaders, end up collectively going a bit mad.
The entire Scarlet Spires, or Eleäzaras himself, go mad.
There's really a lot of choices if you think about it.