With 1 being least boring, I'd say 1.
I've never known a
Katie. (I've known spelling variants of
Caitlin, but none went by
Katie.) So I turn to the popularity statistics. It never got higher than #38. That doesn't put it in the boring category for me. Of course, that wouldn't take into account any Caitlins or Katherines who do go by
Katie, but as I've said, I've never known one, so it doesn't feel boring to me.
That's my defense.
I prefer the
Katie spelling.