Paris Hilton, by far... she became popular for absolutely nothing.
Lindsay Lohan, though I'm not a fan in the least, is still a very new face in Hollywood, and she's probably still learning how to cope with the media and stuff. I honestly don't think that half of what she's famous for was originally an idea she had in mind before starting... probably something that promoters thought would boost fame and stuff like that.
Britney Spears has been getting a lot of heat lately, despite the fact that she had a kid, and tried to slow down her career because of it. The media insisted on following her around and filming every little thing she did, so every "bad thing" or "mistake" she did gets blown way out of proportion (well... most of what she did anyway). But again, I'm not a fan, so whatever...