Calculators are definitely the trickiest to use effectively. They're only okay once you've learned all the parameters.

I LOVE monks. Seriously. Ramza as a monk with Guts as a side skill. Unbeatable.

I alway found Samurais to be useless, though some think they are just the greatest thing in town. I prefer Ninjas, hands down. Two swords > Two hands.

And I still use Chemists for healing. I've never had a priest for healing purposes. I've mastered many a priest, but never found a practical use for one in battle.

Bards and Dancers make for interesting fights, but their songs/dances don't effect enough to actually make the fight easier, generally. Most of the time, when you do a dance that lower strength, it only hits 1 or 2 of the 6 enemies. Then 13 turns later, it hits them again. I'd rather spend that time with a Wizard, using Flare, or a Monk, just Earth Slashing them.

Though I can't say why, I've always liked the Geomancer. Their skills aren't too great, they do little damage, rarely ever inflicting the accompanying status ailment, and their physical damage is on par with that of a squire. But I still do like using Geomancers, and I do quite often.