Probably Mitsuru. She's a heavy direct damage dealer (Ice and Slash) and a backup healer. Everyone else can do what she can (no counting her elements, just her strong points), and more. Her status effects fall on the more useless side of things; while Charm is a good effect, Marin Karin only hits one at a time, and the enemy can still attack *if* Tentarafoo connects.
Ken can heal one or the whole party and can hit with 3 attributes (Light, Lightning, and Pierce).
Akihiko can heal one, debuff whole groups, and hit with Strike and Lightning.
Yukari can heal one or the whole party, un-charm (kinda useless), and hits with Wind and Pierce.
Junpei can defensively buff the party, heals himself per turn, and can hit with 4 attributes (Fire, Strike, Slash, and Pierce).
Aigis can buff the whole party, heal one person, and can hit with Pierce, Strike, and Slash.
Koromaru can evade/hit buff the whole party, nulls two elements, and can hit with Slash, Fire, and Darkness.
Shinjiro is the most useless, but he's only temporary.
Metis can go into Orgia Mode, with no spell and HP costs during it, can hit all enemies with Charm, and uses five attributes (Wind, Ice, Strike, Pierce, and Slash). Gameplay wise, I'd say Metis or Akihiko take the top spot, while Mitsuru and Shinjiro are near the bottom.
Gee, let me think about it. Yeah, I think we have time to find his lost jewelry. Hell, we could find him some matching earrings. ~Ryudo, Grandia II
Best is Akihiko. He has all three debuffs, can be a healer, has the full range of Elec skills with Amp, and can deliver Strike attacks (which is boosted by Fist Master to boot). His stats are ideal, perfectly aligned so that he can make use the best of his abilities.
Worst.. I think Koro just because he lacks the MP for his skills and doesn't have physicals to make use of his high HP.
"Beyond the beaten path lies the absolute end. It matters not who you are, Death awaits you."