If you are looking for ultimate in strength, then you need Zero Gear with four power units.
Omega Weapon, Kingdom Key+ and/or Oathkeeper/Oblivion or the two become one are all the best ones you can have. O/O gives you the longest combos, OW has a Combo Finisher for it's first attack, and KK+ can reach the highest ATK for Roxas.
If your looking for the panel called "Ultima Weapon" which boosts your weapon's strength by 15. It's obtained by getting 255 challenge sigils & redeem it at the moogle shop.