I had a tough time deciding honestly? Like there are so many good keyblades, I love Ultima, Fenrir, Oblivion and Two Become One but my favourite is still.
OATHKEEPER.
While the stats in KH2 aren't really anything to write home about (Ultima/Fenrir are literally the best keyblades in that game.) It's one of the only keyblades that Sora carries that has a meaning behind it, and the design is so cute honestly.