What I use is literally whatever looks suitable for something.
For instance, the 3 characters are a wood elf ranger (using the 3.5 version of the ranger), a tilean mercenary (using Monte's Unfettered class because it more or less exactly works for the guy

) and a 'mage' - actually using the wu jen class, because the character was an Amethyst college student but had reasons to flee at short notice, and since then has been having to advance in his magica studies through a much more intuitive method; hence the spell secrets and the taboos of the wu jen work perfectly. I've dropped the elemental mastery part of the wu jen class, and will give him something more Amethyst magic related instead of the ki focus ability (maybe some sort of speak-with-dead or psionic ability).
I'd like to have a rather different system for magic but I have yet to see one that would fit properly, so just using the pre-existing ones has had to do so far. Plus I make up new and wierd applications of magic on the fly (a necromancer the party faced was able to look through any of the mirrors of his house from any other one, cast spells through them and even move between them. No mechanics, feats or skills involved, I just decided he was able to do this). If I could fins something that would better reflect the nature of magic in Warhammer I'd use it, but at the moment I'm starting to put together a few ideas of how it could be managed. The player of the mage is quite happy for the way his character works mechanically to change at short notice, as long as the actual personality wont have to change