how much are you willing to spend?
Triple Aught Design makes a really good backpack. they are a bit pricey, but really well made.
i have the
Litespeed. fits under the seat in front of you on the plane. still had some leg room. of course, i am kinda a smaller built guy (5'6"), but i have long legs. comfortable to carry, had about 20 pounds in it, walked around most of the day with no real problems.
they also have the larger
EDC pack. not sure if this will fit under seat, but should fit easily in overhead compartment
both backs use the MOLLE system, so you can get pouches to strap on to the pack. MOLLE is military spec, there are a handful of companies that make bags, packs, pouches to strap on to each other. easy to customize to your needs.
other manufactures of similar products (lesser quality, but still good):
Condor,
511 Tactical, Maxpedition. Oakley, also makes pretty good bags that may fit your needs