Enemy Skill Materia is probably the most broken Blue Magic in the series barring XI. Largely cause it gives you the most diverse selection of elemental magic, some of the most broken defensive spells in the series, and very few "only good in rare occasions" spells.

Strago is pretty awesome and has a nice spell selection as well, but he gets most of his good spells late in the game and it's just too easy to sidestep Aero, Tsunami, and Quasar for his ultimate blue mage skill Grand Train. Other than that he's actually not too bad.

Quistis and Kimarhi would be okay if their skills weren't largely weak, mostly situational based spells, and tied to their Limit Breaks limiting their practicality.

Quina and the FFV Blue Mage Class are pretty badass but they are also both Guide Dang It. They are both pretty dominating but a lot of fans miss it, because they don't realize how many of their seemingly situational/useless spells are actually not so useless or situational, with a lot of the enemies and bosses very susceptible to some of the most devastating spells in the Blue Mages arsenal.