What is the Strongest to Weakest keyblades in Kingdom hearts 2?

In terms of attack power, the strongest Keyblade would be the Fenrir, which is obtained after defeating Sephiroth in Radiant Garden.

The most powerfully balanced Keyblade is the Ultima Weapon, which must be synthesized using numerous items received throughout the game. This Keyblade does not have the highest attack power. Instead, it has high magic and combo boosters.

If you haven't got Fenrir or Ultima Weapon, then Decisive Pumpkin is another good Keyblade. Having the same strength as Ultima Weapon and the Combo Boost ability is sweet.