Oh okay, well, sorry about that. I havent played Re coded and no idea as to what keyblades are available. Its nice knowing Ultima and Fenrir are there. Badass keyblades.
Sorry man, fenrir's not in Re:Coded. It's pretty much every keyblade from the international version of KH1 (ie not Diamond Dust and One Winged Angel) plus Zero/One.
And as such the most powerful keyblades are probably Ultima in terms of sheer power of each stroke, but there are other ones that deal slightly less powerful hits but a lot faster. Its very much up to your playstyle to determine which keyblade, and subsequent abilities are most powerful for you. ie I use Ultima or Zero/One which that range extending ability so i can start combos on multiple enemies from a distance.
Also as a side note Oathkeeper is hands down the best defensive keyblade in this game