Illumina does have the highest stats in the game, technically making it the optimum weapon. HOWEVER... I found Atma Weapon hitting for maximum damage way before I got to level 99. Granted, it can never get a critical hit, but when it is already outputting the highest possible damage per swing, that doesn't really matter. The random pearl castings can be a problem in some cases- some enemies absorb it (although not many, only one I can think of is Pearl Dragon), some have a passive Rflect on them, and others actually cast Rflect on themselves. If given the option I'd rather have a weapon that has no spell proc (except maybe Ultima or Meteor) to make it more suitable to any situation.

Oh yeah, and there's no way in hell I would give up the Ragnarok magicite for the Ragnarok sword. All your characters having the Ultima spell > Illumina.