From all over my flist.
Jo Maynard. I love Jo Bettany when she's attending the Chalet School, but after she marries and has supposedly grown up she's immensely irritating. For example, she regards having children as a competition (extra points for multiple births), is rude about other people's babies (she's got red hair, poor thing! he's a bit dark, isn't he?), visits her oldest friend in France and tells her how to rearrange the furniture (OK, the friend is stupid enough to ask for advice on her new house) and makes pointed comments about the friend's long gap between pregnancies.
Nicholas Darrow, Susan Howatch's Starbridge & St Benet's books. I like Nick in Mystical Paths; he's very immature but does recognise there are things he needs to learn. In the later books, when he's an adult, he behaves Much Worse and is allowed to get away with it and still presented as an admirable character by the author. (This whole meme is reminding me of another Howatch novel, where one of the characters keeps saying '...and it makes me want to punch you on the nose'.)
Edmund Bertram. Surely no explanation required...
ajhalluk already mentioned the Weasley twins, but I think there are plenty of other Potterverse characters who are eligible. Albus Dumbledore, for example, or Sirius Black. Or perhaps what I have in mind is a bit of knocking heads together while exclaiming 'stop being so stupid!'.
Guy Charlton from Lorna Hill's Patience/Marjorie books. Arrogant, patronising, sexist...