What I would use:

Dark Knight - I simply love this dressphere, the HP and MP, Defense, Attack, Magic and Magic Defense is simply rox... the only problem about it is that it's agility, really... you know. But I not really use it often

Berserker - Same thing for Dark Knight, but it's attributes are weaker, those Berserker's skills rulez, too. But I often use more DKs than Berserkers (when I use DKs, that is).

Warrior - Yeah, I like the kind of "kill before you're killed" parties as you probably noticed =P. Even if I say those dresspheres above are the best of the game (except Mascot), I just can't leave this thing behind, when Paine learned all abilities of all dresspheres, she was only using this one... I CAN'T LIVE WITHOUT A WARRIOR!!
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Dresspheres I wouldn't use:

White Mage - Even if it's the best for healing, as you saw above, I dislike "weak" dresspheres, so I never bother using this dressphere and I don't like, ONLY to learn abilities (and I was to die when using it ^^)

Songstress - Useless, simply useless. Like Paine: "I hate this dressphere".

Alchemist - Their gun is too big, the dressphere itself is ugly, and I hate my random mix results =P