Energy customers have been warned to look further than "attention-grabbing" low tariffs following the failure of 13 firms since November.
Citizens Advice said its latest ranking of energy suppliers showed a "big gap" in customer service standards, warning that it was often "the first thing to suffer" when a firm was struggling.
Breeze, SSE and Igloo topped the charity's latest quarterly ranking while Nabuh Energy, Utilita and Toto made up the bottom three.
Eversmart Energy would have come third from the bottom if it had not ceased trading earlier this month.
Among the common problems faced by customers of suppliers at the bottom of the rankings were billing issues such as incorrect and late final bills, aggressive debt collection practices, problems with prepayment meters such as credit refunds and faulty meters.
Citizens Advice is urging people to shop around carefully when looking for an energy supplier and recommends comparing companies' customer service performance as well as prices.
Thirteen domestic energy supply companies have gone out of business since November 2018, with six suppliers failing so far this year, affecting more than 300,000 people.
Citizens Advice chief executive Gillian Guy said: "There is still a big gap between those firms who provide an excellent service to consumers, and those who are letting people down.
"Attention-grabbing low tariffs can be a good deal, but we recommend looking deeper than just the headline price when shopping around for the best energy deals.
"We've seen six energy suppliers go bust this year. In our experience, when a company is struggling, customer service is often the first thing to suffer."
