Sky
I'm surprised about Sky. I last had problems about three years ago which came down to damaged wires between the exchange and my house. Sky and BTOpenReach were both very good at getting the problems fixed. Last speed check I performed was about 60Mb/s so the service seems very good.
Maybe customer service quality has gone downhill in the last year but I'd be interested to know how Which conduct their surveys. Is the number of customers polled for each company representative of the relative number of customers each ISP has?