Under these conditions, the strongest faction is Conflux, and after that, it's hard to say, except that Inferno is the weakest (its only chance is to quickly find a scroll with Armageddon, then you can win with Ifrits).
As for personal preferences, I don't like Necromancers (it's just boring to spend the whole game accumulating skeletons, trying to prolong it as much as possible) and Barbarians (it's just a matter of style, I've always preferred hero-mages).
I enjoy playing with all the others :)