Duke Form 3 is as legal as Unlimited Indalecio; yeah, it exists as an alternate SUPER FORM for the FInal Boss, but requires going out of your way doing a side quest to make a boss harder just for one last boss fight.
That said, rank the following:
PCs: obvious.
Alexei: Main villain for a good part of the game.
Zagi: The Seifer of the game; Yuri's self proclaimed rival that you keep fighting over and over again.
Duke: Final Boss with plenty of plot
Yeager: Henchman with decent amount of Screen time, and somewhat memorable.
Adecor and Boccos: They're so god awful its hilarious, thus they feel rankable for the same reasons Solt and Peppor are, cause they pretty much ARE Solt/Peppor MkII...heck, they even have similar designs and tutorial fights based on them screwing up; you certainly fight them enough and see them enough to justify it, and they even have one completely legit fight mid way in the game.
Maybes and what not:
Flynn: He's hard to scale based on when you have him as a PC, and the duel can be lost, though I suppose its totally legit beyond that (its not overpowered or anything...far from it in fact, so its not like a "technically winnable plot fight you're suppose to lose" like early Kratos and Mithos fights, as its very clear they intended "You win" to be a conceivable, regular outcome.) I'm not opposed to ranking him, but he's a bit of an oddball due to the scaling thing.
Barbados (or whatever his name is): Sort of the villain of the first 1/3rd of the game, but has far less plot than Duke or Alexei.
Hunting Guild Trio: They tie into Karol's plot, but...I don't think they quite justify themselves as a ranking idea, but thought I'd toss them out there; frankly, I couldn't care less if they don't get ranked, just figured getting them mentioned couldn't hurt. That said, of these three, frankly? Nan's the only one I'd remotely consider ranking, partially cause it says something that I can't even remember the names of the other two, and Nan definitely had a more prominent/important role than those two.
Droite and Gauche or whatever (the loli Twins) can shove it; I didn't even know you CAN fight them, which says something. Tales side quests can bite me.
Any PS3 exclusive bosses like Don can die too for obvious reasons.
And I'm opposed to any Entelexeia boss; Belius has some plot, but its too brief to really justify ranking to be honest, and she's the best there is. Well, Phaeroh would be fine...except for the fact that you never actually fight him, which sort of makes him unrankable by default.