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. For instance, seats at the bulkhead often have more room but if they’re located near the galley, it could be noisy as the crew prepare meals. The light emanating from the galley could also be a nuisance when the cabin is dark (eye shades are a quick fix). If you like window seats (like I do), you’ll want to check if there really is a window (it could be a blocked window or wall). In the case of Business Class, some airlines have staggered seats which mean that some seats are further from the window (with a storage cabinet between the seat and the window) 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 lavatories and galleys may also not be a good option as you’ll have people bumping into you throughout the flight as they head for the lavatories (or galley for snacks) 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.