I'll rank this based on current skankability.
Hilton was #1 for a long time while Lohan and Aguilera were playing cute girls who were kinda PG13 bad. Now, that's switched. Since Hilton's been in the slammer, she has cleaned up her act quite a bit. She still goes out and parties, but she's been keeping her image low-key.
Aguilera is still playing it smart by keeping her act PG13. Christina's also happily married without kids. She is probably the most effective Cher/Madonna clone to hit pop since the former's decline. I think she just gets a bad rap for playing the act (that's all it is) she does. She's an intelligent businesswoman, a very talented singer, and not a slut. So, no for Christy.
Lohan, on the other hand, is now the top of those three. But, because a girl wants to go out and party and fuck other girls, doesn't make her a skank. There's TONS of dudes in Hollywood who do that crap and don't get the press.
Therefore, my vote goes to one that's not on the list: Britney Spears. First, there was her two-second Vegas marriage to that no-name dude who we've all forgotten by now. Next, this bitch "married" a redneck, idiot, dancer, rapper who is a part time deadbeat dad. THEN she got knocked-up, not once, but TWICE. Once the kids weren't bastards, she divorced K-Fed. That puts her divorce rate at 2:2. None of the other contenders you've listed have that. Her mother's solid trailer trash, and most women take after their mothers. She couldn't wait to show off her C-scar and her unspoilt vag as soon as her personal trainer had removed the baby fat. She's also an absolute moron.
No, Spears is the winner. Hands down.