In BG1, I'd have said Transmuter is THE way to go. Unfortunately, that is no longer the case in BG2. Transmuter cannot cast Spell Deflection, Globe of Invulnerability, almost all the spells that protect you. Illusionist is relatively good - cannot cast Necromantic spell. However, I'd say just do a normal Mage, or mage/cleric or mage/thief if you're going to multi-class. I also like elves in that respect, they have the advantadge of 19 dex. Mage-thief is a very useful character, IMHO - can use bows, thief skills, but can also cast spells. Of course, you have Imoen and Nalia too.
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