Pipex get my vote. As Ozzie says, not the cheapest but very reliable.
On the odd occasions that I've toyed with changing to someone cheaper I've listened to the stories about some of those other ISP.
Admittedly you normally only get to hear the bad stories rather than the good but I've always concluded that it's not worth the risk for £2 or £3 a month.
It might just be a case of you get what you pay for.