Mine was probably a girl I house shared with whilst at Uni. She was totally full of herself and seemed to think that she was god's gift to the world.
She thought that she was extremely good looking, although was very average looking. All she talked about all the time was how gorgeous she was, and about how men would look at her and how many men fancied her. We'd suggest a place to go out to and she'd always say something like 'I can't possibly go there, I get so much attention from men in there'. And we'd be walking along the street and she'd be telling me (and probably telling herself!) that men would be looking at her, even though they didn't appear to be looking at her.
She would also make comments all the time that would big her up but put the person she was talking to down at the same time. A friend of another of our housemates had some boots similar to obnoxious girl's boots and obnoxious girl said 'your boots are similar to mine. Obviously not as expensive as mine though'. She would also tell me regularly that she thought I wasn't very pretty but I had a nice personality so not to worry about it.
Her other thing would be to sit there whilst we were all having a conversation and then suddenly say an unrelated statement about herself such as 'have you noticed how blue my eyes are?' or 'my legs are looking so good today aren't they?'
She actually was like the woman in Friends played by Jennifer Coolidge that Monica and Phoebe didn't like!
Who's the most obnoxious person you've met?