The most comfortable seats in economy class are the seats at the exits because you have all the leg space you need, although the window seat has the bulky door partly in front, so you want to go for the middle or aisle seat. The reason is that only adult passengers without any disabilities and speaking the airline's language are allowed here, so that they can help other passangers in case of an emergency. This might or might not disturbe your sleep. Also it might be a bit smelly after a long flight, although this is seldom a problem from my experience. Some smaller planes have one row of seats with more leg space because there are emergency exits there that lead out onto the wing.
Some airlines have seating plans of their planes on their websites.
Travel agents have seat plans in their computer and can help you find the best seat available. Independent of the leg space and the type of seat you are sitting in, long-time sitting is not good for your body. This goes a long way to make your flight easier to endure. Special meals are given out first and you will have probably finished before anybody else has even started. Ordinary meals are given out from the front of the plane moving backwards.