If you're only going to have one backcountry touring pack, you'd be wise to make it the Dakine Poacher RAS 36L Pack. It's spacious enough to hold a full day's touring kit, but it's still compact and close-fitting to allow for smooth, unburdened descents.
The main compartment offers room for lunch and extra layers, and the back panel access lets you reach what you need without unloading the entire pack. Meanwhile, an avy tool-specific pocket allows quick access to your probe and shovel, in case the worst happens. A padded hipbelt keeps the pack from shifting around while you ride, and the breathable back panel helps keep your back from getting soaked with sweat.
This pack is compatible with Mammut's Removable Airbag 3.0 System (sold separately), which inflates in seconds to prevent you from getting buried in the event of an avalanche. And if you plan on getting into some seriously technical terrain, this pack is also compatible with Dakine's Impact Spine Protector (also sold separately), because you can never take safety too seriously.