Any airline which allows you to select your seats online. When travelling coach you should try to hunt all these seats close to the emergency exits to have more legroom. Itīs hard, and sometimes are blocked by airlines either because they are given to passengers paying full economy or given directly in check in (they need to know if the passenger is fit enough to be able to open the emergency door, and understands the language of the airlines: English in this case). Sometimes these seat can be booked paying an extra.
Generally speaking, I would rather fly any airline with two seats rows either on the left or right if you travel alone, than the regular three seats rows of B777 or B747 which is particularly packed in coach. Less people to disturb to get out if you want to go to the toilet, or be disturbed if you travel on the aisle.
So my choice is either Virgin Atlantic (you should have a look to the premium economy options for discounts when booked well in advance), or B767 on Delta, or Continental.