Energy firms Npower and Scottish Power dominated the last and second last rankings in a Which? survey of customer service provided by UK firms - the bottom position had previously been held by Ryanair.
Which? canvassed more than 3,000 consumers and asked them to rate companies in the banking, telecoms, energy, retail and travel industries on their customer service.
Npower was ranked 100th with while Scottish Power was 99th, dropping from 62nd place last year. Both firms implemented new billing systems this year.
Richard Lloyd, executive director of Which? said that this is a new low for the energy companies who have had to deal with intense criticism from customers over the rising price of bills.
"The Big Six energy companies have now hit rock bottom for customer service and, with record high levels of complaints, it is clear just how far they still have to go to put things right for their customers," he said.
"Good companies know the value of customer service, so it's disappointing that some of our biggest firms seem to have a lot to learn about keeping their customers happy. This survey should be a wake-up call for the companies with the lowest customer scores."
The other members of the Big Six faired only marginally better: EDF Energy was best out of the energy suppliers at 81st place, British Gas and E.On were joint 86th, while SSE was 94th.
On the opposite end of the scale, First Direct tops the chart with 87% of customers giving it a positive score for its customer service. Toiletries specialist Lush, John Lewis, Lakeland, and Waitrose all performed well.