I was once assigned a seat in the middle of a row of five on a Boeing 777 on what turned out to be my longest flight ever: strong head winds extended the flight time of the flight from Kuala Lumpur to Amsterdam to 14 hours 15 minutes!! I can’t remember ever feeling so elated to get off a plane.
Your choice of seat, especially on long-haul flights, is thus crucial for your comfort during the flight. Not all seats are the same. Even in Business/First Class, there are seats that are better located than others. In Economy, seats next to the Emergency Exits usually offer more legroom but beware of window seats next to the Exit as the slide compartment in the door may inhibit your legroom. Seats in the back row often have limited reclining space because of the wall behind. Aisle seats close to a lavatory may also not be a good option as you’ll have people bumping into you throughout the flight as they head for the lavatories and back.
To get an idea of the best seats on the plane, I often look up Seat Guru. This website has a comprehensive list of seating configurations for the aircraft of most major airlines and provides the pros and cons of various seat types.