A writer's job is to think of new plots and create characters who stay with you long after the final page has been read. If that mission is accomplished than we have done what we set out to do, which is to entertain and hopefully educate.
I think that would be Martha, if she counts as "supporting" since she's part of the main cast... If we talk the "recurring" cast, that would be Doc Klein.
My 'definition' to supporting is anyone other than Lois & Clark since they are the MAIN characters of the show. They are the ones the show is about, and there are no episodes where it isn't about one or the other.
