Well, Agrias has to be on the list high up there because you have her for most of the game and is a powerhouse. Also Mustadio (who nobody else mentioned) is extremely usefull for the majority of the game because he can snipe and kill undead 1 hit, without worry of ressurection. I personally feel Snipe is one of the best skills because it immobilizes your opponent.

I'm going to go ahead and agree with eveyone that Beowulf is probably overall the most useful besides Orlandu. He's pretty rocking.

Meliadoul would have been much better if you got her a lot earlier. Getting her so close to Orlandu made her obsolete and useless. Raf and Malak are wastes, and Cloud i feel takes far too long to do the damage that Agrias can instantly. And Cloud must have the Materia Blade to be effective. If it could be any knight sword, he'd be more effective.

As for NPCs. Olan Durai takes the prize. Galaxy Stop is way too powerful. I love it. A shame you only see him in one battle.

So aside from Orlandu and NPCs

1. Beowulf
2. Agrias
3. Mustadio

Reis, Worker 8 and Byblos are all decent but replacable. Later on Mustadio's use fades, but then I make him a ninja and rock people with him that way