Hard question. The best options are probably remnant weapons with associated weapon arts. For example: frostblade (sword-large), nightbloom (sword, midsize), schiavona (spear, midsize). They are also upgradeable. Good choices are also arcane weapons, upgradeable and with learnable weapon arts: highbrand, bluesteel, hawkwind, glaive, demonblade, tataraichi, etc.