I agree with OM: volume is only one criteria; weight is another. You didn't say how much your typical load weighs. I can't use a frameless pack with more than 18 pounds or so, which is a problem because 19 or 20 is as light as I seem to be able to comfortably do for a summer weekend, with a liter of water and 2 days' food. Winter clothes, a relatively dry trail, or a longer trip easily put me at 25 pounds.

I've found the Osprey Exos to be a good pack up to about 23 - 25 pounds, and the Osprey Kestrel and Atmos to be decent to around 30. However, over the last year, I've been switching between Osprey's Kestrel 48 and the Deuter ACT Lite 50+10; I've also used a Deuter ACT Zero 50+15 I borrowed from a buddy. All of these weigh between 3 and 3 and a half pounds, and easily handle a 30 pound load. I've pretty much decided that I prefer the fit of the Deuter packs.

Depending on how heavy your load is, you may find it hard to find a pack that fits well, has a suspension that will handle your load, and weighs less than 3 pounds.