With the number of airlines that now fly to Europe, it will be helpful to recommend the best coach seat airline.
The most comforable flights I have flown were on Swiss Air.
We had a comfortable flight on American Airlines on the Boeing 767 last fall. The 767 seems quite roomy compared to others, an opinion I have also seen on this web sight.
Austrian Airlines ist sehr gut!
I too have had good luck with coach seats on 767's....last fall, a Continental 767-400ER from Newark to Madrid. If you can, though, try Lufthansa...the most luxurious coach class seats I've ever seen!
Last year, for the first time, we flew Scandinavia Airlines from Seattle to Europe. It was some kind of Airbus plane. Good seats. Good service. We are flying them to Europe again this summer.
I don't know about the most comfortable seats, but check out www.seatguru.com for good information on the best and worst seats on each airline's planes.
I also thought SAS had great planes. The seats were really well designed - not only did you have a tray table, but also a mirror and a little net bag to store things (not the seat pocket). Best of all was a separate drinks holder, like in a car, so you didn't have to have the tray table down when you just had a drink and didn't have to worry about knocking it over.
This was from NY to Copenhagen, so I think it was probably an Airbus, but maybe a 777.
I will have to add a third nod to the 767. Dont know how much it varies by airline (I usually fly American), but the 767 is fairly comfortable. as another poster mentioned, check out seatguru.com as well for your particular airline. Good luck!