RyanAir
One index rated RyanAir the second-worst brand in the world for customer service last year. The Dublin-based airline has drawn so much ire from passengers that one set up a website with the URL "ihateryanair.co.uk" and was later forced to forfeit the web address to the company.
RyanAir's bad reputation isn't very surprising considering it once announced plans to charge passengers to use the bathroom (which the company later dropped).
Flying with RyanAir is predictably frustrating in many ways, but the service wasn't so bad that it deterred me from booking flights with the airline while I was living in Europe. The rock-bottom prices keep customers coming back, and I don't know of any airline in the US that lets you fly to another country (or another state) for $20.
I flew with RyanAir several times over the past year, and I did notice some differences from more traditional airlines.