Something old and you don't mind getting ruined. Something warm, very waterproof, mud proof, piss proof, beer proof. If the rain doesn't get you, the drunks will.
Forecast for the entire weekend is dire. And although they have put some flooring down, its not everywhere - only right in front of the stage, so there will be large mud patches. There has been flooding around approaches to the park. They are likely to have put metal walkways down in most places, but the edges and gaps between them are not going to be nice, especially if you are going on Saturday or Sunday after 60 odd thousand people have trampled their way through the previous nights. And lets not talk about the off sight carparking...
Seriously, do not worry about being fashionable. It should be the least of your concerns. Everyone will end up in ultra sexy ponchos, even the most fashionable, by the end of the day. I went to gig on Friday in the downpour at civilised stadium - didn't put poncho on for an hour because I didn't want to look a dick. After two hours wearing it I wasn't any wetter, but because I was damp, I'd got that cold I thought I was in danger of developing hypothermia if I wasn't careful. Its just not worth trying to be cool in those kind of conditions.
Your converse WILL get ruined - do not wear!!! You have been warned. Wellies are pretty much the only way forward. Make sure any waterproof you have, really is waterproof and has hood. Jeans hold water (and mud and piss). Combats or leggings dry much quicker and are much more comfortable. You'll freeze in a dress or shorts later in the night.
If you don't have wellies, Decathlon is your friend. Short cut wellies in various bright colours for £6.99. They have matching good quality (as in not a plastic bag and actually reusable) ponchos for again £6.99. This will be the sexy, cool look of the weekend...
Oh and don't forget the warm, dry change of clothes for the car. You will want to strip in public by end of the night!