NTL gets my vote.
Got broadband installed, asked for 6MB, was told he could only hook up 4MB on the day and would switch me over to 6MB back at 'the office'.
Never got changed over but I was charged for the 6MB connection from that moment on. Thus ensued 8 weeks of me phoning them, approximately 15 hours on the phone, I dont know if they have Joe the Toilet Cleaner answering the phones there but they managed to send out no less than 3 engineers, despite me explaining to Joe in his many guises that all I wanted was to be either switched to 6MB 'back at the office' and refunded what I was incorrectly charged for or left at 4MB and billed accordingly.
The whole sorry sage ended with me sending a letter and an email on the same day explaining in no uncertain terms how dissatisfied I was and how I was considering cancelling the entire contract and suing for the cost of my 15 odd hours of phonecalls.
Within 20 mins of sending the email a nice lady phoned from NTL and sorted out the mess in 3 minutes, yes, 3 minutes. Clearly it wasnt such a complicated ordeal when you got to speak to Mary the accounts lady and not whatever fool they have manning the support calls.