I'd guess the average baseweight among thru-hikers this year is a little under 15 lbs. I'd recommend using a frameless pack for baseweights of 10-12 lbs or less, and a frame pack for over 12. 90% of PCT'ers are using frame packs (almost all internal frame), the most popular of which this year is the finely-made ULA Catalyst (which I am carrying from Kennedy Meadows to Lake Tahoe). Everyone seems to love their ULA packs.
Behind the Catalyst in popularity is probably the Golite Pinnacle, which is one of the few frameless packs that can be bought retail. Other popular packs are others in the ULA line, the Golite Jam2, the Gossamer Gear Mariposa Plus, Granite Gear packs, and occasionally some Osprey, Six Moon Designs, and other packs. I have only seen one other person ("Lint") with another Mountain Laurel Designs pack like myself. I highly recommend them. I have not seen anyone with a lighter backpack than myself (not the baseweight, but the backpack itself). I have met one person with a sub-8 lbs baseweight, and she was a girl ("Cat").
If you can get your baseweight minus pack to under 10 lbs, then you can switch to an ultralight frameless backpack and instantly drop another 2 pounds (a kind of snowball weight-dropping effect). Then, you can walk more miles and have fewer days between resupplies, meaning less food to carry.
I recommend the well-built and minimalist Mountain Laurel Designs and Zpacks backpacks. Their translucent cuben fiber packs are the lightest reasonably durable packs on the market (weighing as little as 6 oz). It may be too late to order an MLD pack for a trek this summer, but Zpacks is accepting orders through the end of MAY and has a turnaround of about a week. If you've got a low baseweight with a 2 or 3 lbs pack, getting a crazy light Zpack mailed to you on the trail is a quick way to drop 2 lbs. The Golite line weighs a pound more than the truly ultralight packs and are not as carefully designed or sewn, but should be sufficient for most hikers. Used ultralight packs can be found at the backpackinglight.com Gear Swap forum. I have bought many things at this forum.
When you're already out on the trail, it may seem like a daunting task to get ahold of some ultralight gear and trade out your equipment, but it can be done. You'll need to take some time online to peruse and order gear and arrange to have it mailed to a resupply point a couple weeks up the trail. You'll probably have to make some phone calls to clarify the shipping address and method.