I'm sold on Arc'teryx's Women's Briza 75 ($375; arcteryx.com). Here's why: There's nothing worse when you're cold, tired, or turning blue, thanks to an unexpected thunderstorm, than having to rummage around in your pack for the long underwear that disappeared into a black hole. With its full-length side zip, the 75-liter Briza allows you to reach in anywhere and instantly locate the really important stuff, like your emergency chocolate stash or the raingear that you should have packed on top to begin with. You can tell Arc'teryx made Briza with an experienced outdoorswomen in mind--it offers a lot of must-have bells and whistles like a zip-out compartment divider, a removable top lid, a hydration bladder pocket under the lid, and a special sleeping bag compartment, all of which make life practically luxurious when you're roughing it in the woods.
Option two is REI's Venus 75 ($200; rei.com). In addition to its pretty blue-and-lavender color scheme, the Venus has everything you'll need to go long. Like the Briza, the Venus has a zipper running down the front. It doesn't run quite the length of the whole pack, which makes the opening feel a bit like a duffel, but it has a ton of great aspects: zippered pockets allow you to store small items like Clif Bars and keys, the top lid comes all the way off for easy storing in your tent, the shoulder strap system is designed for a women's shape and easily adjusts with "rip-and-stick" technology, and the lash points and straps are virtually everywhere, so you can weigh yourself down with sleeping pads, ice axes, or whatever you need to stay comfortable.