Mages rocked in BG1.
Insant-win buttons on every spell level.
1: Sleep, grease.
2: Horror, web.
3: Haste, fireball.
4: More complicated spell level, but combining greater malison with horror for instance was BRUTAL, also, imp invis was quite brutal in the right circumstances.
5: Actually, the one spell level where I was quite dis-satisfied in BG1, none of the spells where very interesting there.
In regards to the topic, I haven't tried druids, but my two wizards are definitely deciding factors in both normal combats (aoe is extremely effective towards weaker enemies) and in boss battles (slick is basically instant win button, just lacks a long duration).
It might be that druids are too good atm?
Maybe I'll test them at some point.