My favorite is [name]Catherine[/name]. It looks the most elegant to me, I love that it’s the French spelling, and (from a visual standpoint) I prefer the smooth openness of the C to the prickliness of the K.
My second favorite is [name]Katharine[/name]. It’s the spelling that’s most true to the origins of the name, and Hepburn is a great association. My biggest problem with [name]Katharine[/name] is that it’s so close to the far more popular [name]Katherine[/name], I think people would carelessly get it wrong most of the time. I prefer to save this spelling for use as a middle name.
[name]Kathryn[/name] is streamlined and has a kind of mid-century spunkiness the other spellings lack, so I like it well enough. [name]Katherine[/name] is my least favorite of the four just because I find it the least interesting.