BT
Worst customer service of any agency, business or organisation that I've ever had to deal with.

Long telephone queue times followed by inflexible, disconnected call centre checklists given to you rote by someone who speaks English well enough to go through a call centre checklist but not one whit more.
( WormuIus, Wed 5 Oct 2011, 13:35, 2 replies, latest was 14 years ago)

Best service I've received has been Natwest Bank

( WormuIus, Wed 5 Oct 2011, 13:38, Reply)

My job involves a good deal of direct correspondence and co-operation with BT
Roughly speaking, time spent breaks down thusly;

30% waiting for BT to pick up their fucking phone
20% apologising to customers for delays directly caused by BT
45% banging on about how utterly shit BT are
5% listening to our manager sollioquise about how glad he is that he no longer works for BT
(Darth Foxtrot A one-man army dedicated to making fetch happen, Wed 5 Oct 2011, 13:40, Reply)

Perhaps I'm biased because I'm having internet problems at the moment.

( WormuIus, Wed 5 Oct 2011, 13:42, Reply)

Perhaps
but only in the same way that my opinion of them is biased by their constant inability to do a good job
(Darth Foxtrot A one-man army dedicated to making fetch happen, Wed 5 Oct 2011, 13:44, Reply)