From your experience obviously..
Best for me by a mile has gotta be sky. More expensive than the rest it seems but customer service is excellent and internet doesn't randomly go off or be capped
Worst would have to be BT. They tried ripping me off ~£300..gave me a bill for that amount and refused to give an itemized one when I informed them that there was no way on earth that I owed that much. Reckoned they would send me itemized bill AFTER I paid the £300.
Also had loads of problems with them cutting internet off (technical error was always the excuse) and seemed unable to speak to anyone who knew anything about anything...for example internet died one day, and the person I spoke to suggested changing my saved homepage? Despite no pages working at all..changed saved homepage wont bloody work as I told them, and they said to try it anyway