My vote for character who was least utilized would be Cat. There was a real opportunity to use her to grow Lois as a person, to bring them together as friends after they started off as almost enemies, but that didn't happen. I'm glad some writers have taken the character and done so many nice things with her.
My favorite supporting character to write is Lucy. I'm not sure why, but she has so much potential. She's smart like Lois but not driven to succeed - in fact, she's almost driven to prove to her parents that she's not going to succeed no matter what they do or say! And writing her so that she grows through that to being a mature adult is a real challenge, but for me it's worth it.
(I have an alt-Clark WIP where Lucy is quite mature. I may pull it out and polish off at a later date. I think my muses have had their interests piqued.)
And I'm the only one to vote for Lucy as my favorite (so far). Poor kid. She always gets the short end of the stick.
