My personal favorites are Oathkeeper, Oblivion, and Destiny's Embrace, but I feel like the very last of the three is the best of them. Oathkeeper is nice(and is where most of my game accounts got their names), and Oblivion's nice and dark to counteract it, but I actually own Destiny's Embrace, and it's really good. A nice, complex design, counteracted with the lovely colors that most certainly suit both wielders.
My least favorite is probably either Fatal Crest or Wishing Star. Fatal Crest just looks like they were trying too hard to make it 'cool,' but Wishing Star is just.... bleugh.
So the best/worst in my book are Destiny's Embrace and Wishing Star. So much for my aesthetic. But Allwil13 is right-- a Keyblade is a Keyblade, however bad it is.