So may top choices missing from this list, like the packs from ULA. Not much point in adding any now since these polls don't take into account poor initial choices and votes will not be weighted properly.
Really a function of $$$ and if you are long distance thru hiking AND if you are an Ultra-lighter - pearly makes a great point as well and some of these are way bigger than I would choose to carry....ever.
One thing that should be noted is that a pack isn't just the sum of its parts. You need to know if it'll fit you, and by that I mean height, back length etc. It will make a lot of subtle differences that during a long trek will add up and ultimately make you uncomfortable. Do the research!!
I finished backpacking for 2 months around Europe with the Atmos 65L. I looked like the biggest tourist but the ventilation really saved me in the heat.
Ahhh... the current business incarnations of two of the top designers/pack makers of a few decades ago- Patrick Smith (formerly of "Mountainsmith) and Dana Gleason of Dana Designs. Both of these gents remain dedicated to bombproof, lifetime gear- and packs that make carrying a heavy load as workable as possible. With the ultralight craze proving to be a bit more than a fad their old packs don't receive as much respect as they deserve, but they're excellent. If you're a big guy like me (6'1, 250+) a pound or three in pack weight doesn't make much difference in how your day goes, and worrying about ounces quickly seems foolish. That said, for fit folks doing big miles or those with significant injury issues I completely get going ultralight. In years past on long treks I comfortably carried up to 75 pounds in Mountainsmith and Dana packs for significant distances and trip lengths. If I was going to do that again, these are the folks I'd turn to for a new pack- except that my 20+ year old Dana and 25+ year old Mountainsmiths still get the job done. Kifaru is all US-made, Mystery Ranch has some US and some overseas manufacturing.