Fighters definitely.
Have you tried taking out a party of goblins with a bunch of 1st level Wizards? It doesn't work very well.
Seriously, though, IWD is so hack-n-slash oriented, I prefer a fighter over a wizard. Having great armor is a must. THACO is essential. It takes a long time for a Wizard to reach a high enough level to be effective in battle.
------------------
Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.