The Conjurer is quite good, since you only miss one really usefull spell: Identify (as Volever already stated), but you really don't have to identify every item and the items you do want or need to know can be done in shops.
However, in the end I probably also prefer the Illusionist: I like the personal image of this chanting circuswizard more than that of a conjurer (probably because it makes me think of Edwin, who I find mean

), and you hardly miss good spells (Skull Trap can be interesting, and Horror can be quite usefull but you can easily live without it). So in the end, I join in with Volever's opinion: I would advise a Illusionist the most, or else a Conjurer. And I wouldn't take the challenge yet, by playing a Diviner or a Invoker; it makes you miss some very nice spells like Monster Summoning and Flame Arrow. I'm having Dynaheir in my party right now, and that's only working since she has a great status. But I wouldn't advice it for your Leading Character, it's too soon.
And finally I would also note this: it is nice when your mage has a high dexterity, so that he has a low AC (=high defence) and can better avoid enemy hits. Though it is not neccessarily that you give that to your character at the beginning of the game, since you will find Bracers of Dexterity at the Gnoll Stronghold, which will turn your dexterity up to 18 (while wearing the bracers). So you could instead focus on your constitution, giving you a bit more hitpoints which is usefull, since mages don't have much of those.