Firstly, AOL are the spawn of Satan himself and should be avoided like the plague. Atbout 8 years ago the US radio host Thom Hartman played a recording of a phonecall between a customer and AOL who wanted to cancel - see for yourself what happened :
http://www.youtube.com/watch?v=xmpDSBAh6RY - Hartman himself admitted that in order to cancel with them, he told his credit card company that his card was missing and should be cancelled, just to stop AOL taking any more money. My experience wasn't quite that bad, but is was pretty rubbish. Frighteningly, AOL still make decent money out of dialup for rural areas in the States. I think they should be driven out of business, perhaps by a mob armed with pitchforks and flaming torches.
As for the rest, unless they have their own network (Virgin), they are all using BT Wholesale. If you go for Sky, remember that Sky will try and sell you the rest of their package, which is much more profitable for them. I had a terrible time with BT, and after months of waiting for them to connect me up, I cancelled. I think they are one of the most disfunctional companies I have ever dealt with. On the other hand, my brother (who is a senior IT person at an insurance company, and therefore needs
really fast and reliable broadband) thinks they are great, and switched to them from Virgin, who he hated. Virgin are my providers, and I dont really have a problem with them, but like some others, my fibre box is opposite my house, so I tend to get a very good speed.
Frankly, if you ask anyone, they will all have horror stories, and thumbs up, and pretty much all the ISP's will have a mixture of both. If its OK, we dont really think about it, but if it goes wrong, it tends to stick in the mind!
If your in a rural area, then as British Red said, its BT all the way. The Public Accounts Commitee had a go at them last year for being incrediably slow and expensive in rolling out rural broadband, which is being paid for with taxpayers cash. Some of my customers have turned to dish based systems, just to get something half decent.
In addition to Idleknights suggestion of broadband.co.uk, Brianist of UKFree.TV
http://www.ukfree.tv/txdetail.php?a=NY299425&irt=760861#b760861 recently suggested thinkbroadband.com to a similar question.
Good luck!