Backpacks
The sole purpose of your pack is carry your gear and supplies comfortably. That's all. Keep that in mind when selecting a pack. Overbuilt packs are the norm at most sporting goods stores. Many backpacks found in stores weigh more - when empty - than all of some PCT thru-hikers' gear combined.
Lightweight materials are sturdier than they look. Your backpack doesn't need to be bombproof. You're going to a national scenic trail. You are not going to war. You aren't going out to bushwack through the Amazon or build a trail through the Sierras. You are going out for a stroll along the PCT. Choose your pack accordingly.
You may want to choose your backpack after you've chosen most of your other gear. That way you'll know about how much volume your pack will need, and you'll have some idea of how much weight your pack will need to support. Then you can go buy the lightest and smallest pack that meets your needs.
Framed vs Frameless
The average thru-hiker has some kind of frame built into their pack. Ultralight hikers carry such a light load that they can often get by with a frameless backpack, but most hikers prefer a backpack with internal frame support. If you think you'll be carrying 30 pounds or more (including food and water) on a regular basis then you should consider using a backpack with an internal frame.
The Lightest Option
The Gossamer Gear Whisper Uberlight Pack weighs in at just 3.7 oz, but it's 2,000 c.i. volume may be too small for all but the most minimalist hikers. To learn more about the Whisper Uberlight Pack, click here.
If you need more space, durability, and the ability carry more weight, the Ray-Way Backpack ($55) has a volume of 3,700 c.i. and weighs only 9.5 oz. The catch is: you have to sew it yourself. To learn more about the Ray-Way Backpack, click here
The next lightest pack with enough volume for a thru-hike is The Prophet by Mountain Laurel Designs. It has a max volume of 2,900 c.i. It weighs 14.5 oz (but can be reduced to 12.5 oz by removing various straps). You don't have to sew it yourself, but it costs $180. To learn more about The Prophet, click here.
The Cheapest Option
Internal frame backpacks can be found everywhere. Try to find one that weighs less than 4 lbs. You may start at thrift stores first. Check Craigslist. Ask your friends and family. Anything is cheaper than buying new, and internal frame backpacks are very common.
The cheapest framless option is the Ray-Way backpack kit ($55). You do have to sew it yourself (which is why it is so inexpensive). It is possible to make one yourself without the kit for about $11 worth of materials if you are good at that sort of thing. Other frameless backpacks are substanially more expensive - unless you can find them on sale.
Less Common Options
Hip packs are packs supported solely by your hips. They have no shoulder straps. Though they come in all different sizes, hip packs genrally have very limited volume. Hikers who use only hip packs must reduce thier gear volume substantially, and resupply more often.