General consensus says Leviathan Blade: Undead bonus is nice, but the jump forward on Cold Iron's third hit is too much of a liability.

People say the Elemental drivers are good, with the exception of the Prisma Driver. Valiance is always a solid option. Iron Slug or Callahan are also both rad choices, their range being their main asset in either Piercing or Normal damage varieties. Antigua series is also good, but split damage is not always preferable. Honestly? See if you can't try out their base forms, see which one you like, and go from there, they're all good choices.