The survey by consumer watchdog Which? found the airline received a refund satisfaction score of 47%, the lowest of all eligible airlines.
Over a fifth of the survey's respondents who were eligible for a refund due to COVID disruption said it took more than a month for Ryanair to reimburse them.
"Ryanair is the most awkward airline to deal with that I have ever come across," one respondent said. "It seems to be proud of being difficult."
"Ryanair’s consistently terrible customer service has made it a fixture among the worst performers in our surveys for many years – but the airline plumbed new depths with its handling of Covid refunds," said Which? Travel editor Rory Boland.
Just above Ryanair with a customer satisfaction score of 63% was British Airways. Many of the complaints regarding BA were in relation to poor customer service on their phone lines, with respondents reporting being put on hold for unacceptable amounts of time.
By law, passengers affected by flights cancelled during the pandemic were entitled to receive refunds within 14 days.
Jet2 was found to be the most efficient of the airlines in handling refunds, scoring 84% in the survey.
"The pandemic has seen Jet2 shine," a respondent said. "Its standard of customer care exceeds that of any other low-cost airline."
