Most characters can be beefed up to be super awesome, regardless of their class, but what characters flat out rock the house (or are easiest to make super powerful)? I assume it's purely based on unique evilities?
Majin isn't that great imo. really there isn't any hands down greatest. it depends on what oyu need them for. Alot of people like teh Masked Hero for Item world runs.
Adell is a really popular Unique Character but he is DLC Thursday has the highest stat growth of all characters (base 155% for all) also DLC Aurum and Gig are both monsters with 3x3 attacks Ninjas can dodge skills and Magic really well.
Honestly it really just depends on how you set them up since you can transfer most evilities it makes most classes fairly even
"Carter, hand me my thinking grenades." Peter (Family Guy)
Shrine Maiden with an Axe is something to behold. 30% damage increase when hitting one target, a weapon that has two S-rank, single target moves... Hmm. Then you have a free evility slot to play with.
Don't ever even consider making a Majin for end-game outside of extracting their evilities.
The dragon DLC has, in my opinion, the best unique move in the game, a 5 square plus shaped A power move with a range of 5, making it (as far as 2.0 will allow me) the only multi-target A-rank non-magic spell.
Taro and Yukimaru have the strongest specials in the game. Taros' is an S-rank single tile non-elemental move that can be powered up to over 700%. Yukimarus' is the same, with an ice element added.
Of course, you can still do what ever you want to do. For example, I have Beyond X tearing everything up with Big Bang and dealing more damage to enemies than it's possible to have HP.
Don't you know I'm all that and a bucket of wings?
Gig is probably the single most useful character so far for me. He has a 3x3 attack, decent movement and can fly through enemies. That makes him an excellent Item World sniper so your other runner can get through whenever there's a Gate Guardian.
Hero Prinny is also very useful for his ability to revive at the base panel every turn.
I don't use any story characters, simply because they cannot reach the base stats that generic characters can. All of my generic characters have been captured from LoC floor 99 in a rank 40 item will all stronger enemy bills passed. (capture a few enemies, exit, re-enter)
Those guys generally have better base stats than story characters could dream of. The downsides are that you cannot reincarnate them (you can, but they'll lose those stats and you cannot get them back), so you are limited in how you can build them, and they often come with strange skills. Like my Galaxy Skull has all Axe skills +9, but no spells. But my female Samurai had all the Omega Spells. But that's not that much of a big deal, you can just learn the skills you want yourself.