The big six energy companies all rank poorly in the Which? annual customer satisfaction survey. The results come as more consumers are turning to small and medium size suppliers.
Utility Warehouse, Flow Energy, Octopus Energy, PFP Energy and Utilita took were the highest ranked companies in the survey of 9,000 consumers.
At the bottom, Scottish Power and British Gas took joint 26th position, below SSE at 24 and EDF and E.On in joint 22nd position.
Only 32% of customers with the Big Six are very satisfied on average, compared with 52% for medium-sized suppliers and 45% for smaller suppliers.
Which? made just one energy company a "recommended provider" this year - the small supplier Octopus Energy.
The survey found customers with the Big Six on a standard variable tariff could save up to £333 a year by moving to the cheapest dual fuel deal on the market.
"Energy customers shouldn't tolerate shoddy service, sky-high prices or failure to resolve complaints,” said Alex Neill, Which? managing director of home products and services.
"Once again our survey shows many of the biggest energy companies languishing at the bottom of the table, with some small suppliers showing signs of growing pains.
"Energy companies should be working to give their customers a fair deal, which includes competitive prices and good customer service. If you're not getting a fair deal you should switch away today to potentially save hundreds of pounds and get better service."
Npower said: "It is disappointing to see these results as we're currently receiving the second lowest number of complaints per 100,000 customers out of all the Big Six energy suppliers and our own internal customer satisfaction also shows we're improving.
"Clearly, there's more for us to do and we'll continue working on improving our position in this survey."