The supplier has sat at the top of the table since the end of 2012.
It received 306 complaints for every 100,000 customers - almost ten times as many complaints as received by its big six rival, SSE, which sat at the bottom of the complaints league table with just 31 complaints made per 100,000 customers.
The figures mean that nPower has seen its complaints increase by 300 per cent in the period from January 2012 to December 2013.
A considerable amount of complaints it received related to its billing system, which it has acknowledged was causing problems. Amongst the problems encountered by nPower's customers were new accounts not being set up, late arriving bills and payments being stopped.
Commenting on nPower's results, Gillian Guy, Citizen's Advice chief executive, stated:
"Things are getting worse not better for nPower customers. It is unacceptable that nPower has not yet sorted out the serious failings in its billing systems and customer service which are causing so many complaints and serious problems for its customers.
"For well over a year now some nPower customers have been finding their finances thrown into chaos. Some are not receiving bills and others are ending up in debt because their Direct Debit was cancelled. Citizen's Advice has asked nPower to make sure people affected get any appropriate compensation," she added.
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