Toss up between Blue Mage and Time Mage. Especially FFV/FFT Time Mages, they are just brutal. Blue Mages can be a pain to get their skills until you really learn where they all are. I don't even use Green Materia in VII, the Enemy Skill Materia is just superior and by the end of Disc 1, you'll have most of the broken skills.

Quina is also a beast but sadly you need to know your enemies well. Though by Disc 3 it'll have access to spells like Tornado and Quake which do some serious damage. I think Quistis is my least favorite Blue Mage but its because her skills are outclassed by everyone's limit breaks. Strago is surprisingly powerful once you get most of his Lores and FFV Blue Mages are just plain mean.

XI's Blue Mages sound pretty frightening as well.