It depends what you mean by "strongest" Overall I think both Balthier and Cid are monsters in their own classes but I feel both are hindered by being so dependent on their default classes. Both have damn good stats though but I feel Ramza is just way better overall since he has a greater level of versatility especially stat wise. I mean if you ever need to know how good a class is, just make Ramza into that class.

Then again, its due to Ramza being made not to have a true default class. Cid and Balthier are incredibly good in their own rights but who in their right mind would transform Cid into a heavy mage (and remove all traces of his original class) or transform Balthier into a Mime? Its not that Cid can't be a good mage and with Balthier's stats he would make a damn good mime but most people go through the other classes to help build on their default class not to exploit their good stats. I know Ramza's Squire class is anything but fair but even with its strengths its not enough to overlook other classes *dreams of unlocking Dark Knight...*

Cid is still the best when dealing with multiple opponents but Balthier is more effective in making sure a specific enemy dies this round. Barrage is powerful but its mostly a one target deal unless you set it up right Hallowed Bolt is everyone's friend. Then again Cid is a killing machine whereas Balthier's Sky Pirate class is overall useful and broken (an evil mix of Squire, Thief, Machinist, and Ninja) so your options in battle are plenty...

Basically:

Balthier = Highest damage to single target
T.G Cid = Highest damage to multiple targets
Ramza = Overall versatility