I always debate this question. I usually like getting a window seat so I can look out the window and lean against it when I want to sleep. I like the aisle so I can get up without bothering people, but I feel cramped in an aisle seat. The reason is that I have to share the armrest with the person next to me, and I have to keep my aisle arm close to me or else I get bumped when people walk next to me. I guess the aisle is best when I have an empty middle seat next to me. I had the right middle section aisle seat on a UA 747 with no seat mate. That was great.
My favorite seat is a E+ window bulkhead seat in a mainline UA Airbus 319/20. There's so much room that you can get out of your seat without your seatmates getting up.