Personally I would say Jet2, Tui and Ryanair in that order.
Things have improved recently at Jet2 and Ryanair but as the post above says Ryanair still have a way to go. Tui seems to be in a state of flux right now aircraft wise maybe with a lack of aircraft and Jet2 seem to be going towards Airbus which might be a shrewd business move. I worry that Jet2 might be biting off more than they can chew sometimes but you have to be aggressive I guess in business, look at Ryanair.
Just my thoughts based on some experience.