To see many more of the best backpacks ranked by weight and other characteristics and features check out the Lightweight Backpacks List.
Best backpack for backpacking is a tricky idea, but I've judged best to be based on several things including weight, reputation, and features, and I've divided things up in to a few categories to help you find the kind of backpack that you're looking for.
Best Backpacks - Frameless
ULA Equipment CDT - If you've spent any time around lightweight backpackers you've probably heard about ULA Equipment. Unlike many lightweight manufacturers that dabble in a few different things ULA is focused almost 100% on backpacks, and that focus comes through in their products. The CDT is light and low capacity, perfect for those going lightweight backpacking and a serious contender for best backpack.
GoLite Jam - The Jam is not a new backpack by any means. It's been around for a long time and has gone through a few evolutions in the process. The latest version of this lightweight backpack is a touch heavier than some of its predecessors but the new weight is arguably worth it for a better backpack.
Terra Nova Equipment Laser 35 - The Laser is a bit of a stand out. Unlike most backpacks it's a front loader, meaning that you have easy access to everything in your backpack, so if front loaders appeal to you definitely consider the Laser. It's made by a UK company not well known in North America, but with a well deserved reputation of quality in the UK.
Gossamer Gear G4 Ultralight - The G4 is a lightweight backpacks that, like the Jam, has been around for awhile and has been through a few design tweaks in the process that could put it up there as best backpack in the frameless backpacks category. And it's made by Gossamer Gear, another well known name in the lightweight backpacking community that you wont be disappointed with.
Mountain Laurel Designs Exodus - The Exodus by MLD won the Backpacker Magazine award in 2011 for Readers Choice of Best Ultralight Backpack. Like the other frameless backpacks on this list it's well known and well loved by the lightweight backpacking community.
Best Backpacks - Frames
ULA Equipment Circuit - The Circuit is similar to the CDT from the frameless list above, the difference between the two being a lightweight frame and a larger carrying capacity for the Circuit, making it an ideal lightweight backpack for those looking to go lightweight with a frame backpack.
Osprey Atmos 50 - While not as light as the Circuit or the Mariposa below, the Atmos is a sturdy frame backpack made by one of the better known brand names in the backpacking world. It's also very popular and well liked by the backpacking community, and makes a fine choice for those not as concerned about weight.
Granite Gear Blaze A.C. 60 (Women's version)- The Blaze A.C. 60 is a new backpack for 2011 from Granite Gear. It's made a big impression quick, grabbing top honors from the Backpacker Magazine Awards for Editor's Choice of Best Backpack.
Gossamer Gear Mariposa - Gossamer Gear is another backpack manufacturer that focuses on making a durable yet lightweight backpack, and the Mariposa fits that description well. Gossamer Gear backpacks are common with lightweight backpacking enthusiasts out on the trail and the Mariposa is one of their most popular backpacks.
Best Backpacks - High Capacity
ULA Equipment Catalyst - It's no mistake that ULA has a backpack in each of these three categories. They make some of the best backpacks out there. They make an excellent product, and while the Catalyst is on the smaller side of the backpacks in the high capacity list it also weighs far less, and if weight is a concern the Catalyst is probably the backpack for you.
Osprey Argon 110 - Osprey makes a whole line of Argon bags, including a 70, and 85, and the 110. If your best backpack is the one with the capacity to carry all your gear, it's difficult to match up with this backpack.
Arc'teryx Bora 95 - The Bora 95 is another top of the line backpack. You wont be saving on weight with this one, but it's got all the features and capacity to carry a wide variety of gear, and if you need the space to carry all your gear definitely consider the Bora 95.