I'd have to say Martha and Jonathan. It's hard to write them without seeming so country hospitality, "aw shucks," etc., when in reality they should be critical of some of Clark's decisions and, let's face it, parents have moments where they're really disappointed in their children, even adults, and aren't too thrilled to see them.
The fic I am writing now has Jonathan being VERY unsupportive of Clark's lifestyle, and I have had to really stop and think as I write him to make sure there is a fine line between his love for his son and his disapproval of what Clark is doing without being over- or underbearing.
