I would suggest a Chaotic Neutral mage (any race will do) for a challenge. Number one: mages die at a swat of a blade; Number two: their spells have a high tendency of getting disrupted during an all-out battle, which render them rather useless suicidals; Number three: getting spell scrolls cost you most of your meager fortune.
But mages do have very cool distructive abilities if they get to cast their spells.
I'm currently working on refining my elven mage on my fourth IWD round.
For the most superior character, use either a human Fighter or Paladin. Human fighters were the only characters I managed to keep alive from the beginning of the game to the end in my first IWD round (maybe it's my violent nature which jives so well with fighters, but um ...*
Note that once a Paladin gets to level 20, he is a lean, mean fighting machine who can effortlessly hack and pave a bloody path through armies of baddies for the rest of your party. Now is that superior or what?
"Step right up! I've got plenty of deaths for everybody!"