Ramza is great as a Samurai/Squire. Samurai gives him heavy armor, two-handed (double physical damage), ranged offensive magic that won't harm allies (like summons), ranged healing magic that won't heal enemies (like summons). Squire lets him boost his own stats and give a boost to allies. Cast haste on him at the beginning of battle, have him boost his stats, and then unleash him on the enemy. Anything that gets in range is fucked, anything dumb enough to get in melee range doubly so.
Agrias as Holy Knight/Samurai isn't bad either.
Mustadio as Engineer/Knight can be a NASTY combination. Ranged attacks that can break an enemy's armor, weapon, helmet, speed, etc., he can render them motionless or unable to act from a distance, and he can instantly petrify undead. He's fucking awesome as a support character, and too many idiots think he's useless.
Add a Ninja/Thief for a glass cannon that can swoop in and backstab the enemy with dual weapons, or someone who can steal all the shit off someone Mustadio crippled, and a Summoner with some other magical support class, like black, white, or time magic. Cid is so overpowered, adding him to your party puts the game on easy mode. A fully-powered Calculator run by the AI is an all-power god of war.
