The consumer group spoke to more than 8,900 energy customers for the latest of its annual satisfaction surveys.
For the sixth year running Npower scored bottom with a satisfaction rating of 41 per cent. It was followed closely by Scottish Power on 44 per cent and The Co-operative Energy on 45 per cent.
SSE scored 52 per cent, only one per cent shy of the survey average. Three of the big six managed to meet or exceed the average, but only just, with E.ON on 53 per cent, British Gas on 54 per cent and EDF topping the table on 55 per cent.
A small supplier took the top spot for the fifth consecutive year: OVO Energy came first with 82 per cent, Good Energy came a close second with 81 per cent and Ecotricity third with 77 per cent.
Which? executive director, Richard Lloyd, said: “Once again the smaller energy suppliers are leading the way when it comes to customer satisfaction, leaving the Big Six providers in their wake. With energy customers facing big bills this winter, energy companies need to up their game and provide the service their customers deserve.
“We hope the competition inquiry into the energy market proposes radical ways to shake up the energy market and find ways to ensure millions of people can switch to better deals while also protecting vulnerable customers from paying over the odds.”
Responding to the report, Energy UK said in a statement: “Customer service is very important for energy suppliers. Companies work hard to get things right and, when things do go wrong, most complaints are dealt with by the end of the next working day. The majority of complaints are about billing and the roll-out of smart meters will improve this.
“There are now 34 suppliers operating in the market and there are deals and services to suit every customer so it pays to shop around. Almost 4 million people switched in 2015 and 338,253 in December showing that it is now quicker and easier for customers to switch.”