Passengers gave the Dublin-based carrier the lowest possible rating when it comes to boarding, seat comfort, refreshments and cabin environment, according to a survey.
This left it with an overall customer score of just 40% in the Which? poll that questioned 7,901 people based on 12,459 flying experiences.
Of those who said there was one airline which they would never travel on, more than two out of three (70%) named Ryanair.
Other names low down in the rankings for short-haul flights included Thomas Cook Airlines (52%), Wizz Air (54%), Vueling Airlines (54%) and British Airways (56%).
EasyJet came in the middle at 63%.
Image: Jet2 was among the best-rated airlines in the poll
Aurigny (81%), Swiss Airlines (80%) and Jet2 (75%) gained the highest scores for airlines making short-haul trips.
Jet2 impressed passengers with its service, seats and boarding.
In December the Civil Aviation Authority launched enforcement action against Ryanair over its decision not to pay compensation for flight disruption resulting from industrial action by its staff during summer 2018.
The airline has also faced criticism for changing its hand luggage policy twice last year, leading to charges for passengers flying with small wheelie suitcases.
One passenger told Which?: “There are too many rules. I worry about getting caught with hidden costs.”
A Ryanair spokeswoman said the research did not take into account the cost of fares – which is “the single most important factor for UK consumers” – and described it as “totally unrepresentative” compared with its 141 million annual passengers.
She added the airline‘s average fare of £35 is “a fraction of the high fares charged by Which‘s ‘recommended‘ airlines”.
Rory Boland, editor of Which? Travel magazine, said: “Airfares might seem to be getting cheaper, but only if you don‘t fancy sitting with your family and children or taking even a small cabin bag on-board.
“Increasingly you need a calculator to work out what the final bill will be, especially with Ryanair.”