I know I made a top 10, which isn't what you wanted, but I still have a #1. Diamond Dust has my favorite design. I might be overly fond of it because of wanting to play KH1FM for years before we finally got it in the U.S.
1. Diamond Dust
2. Sora and Riku's Combined Keyblade (3D)
3. Ultima Weapon (KH1)
4. Olympia
5. Oblivion
6. Pumpkin King
7. Ever After
8. Crystal Snow
9. Oathkeeper
10. Three Wishes
It's hard for me to remember the Keyblades from the side games, so I had to use Google Images to remember some of them. And I've never played Union Chi. I saw some swirly blue flame Keyblade (there was a star at the top inside a circle) that I couldn't place.