The Arc'teryx Cierzo 18 is one of the lightest packs in our test. It feels like this pack is designed for a fast-and-light alpine mission where it's okay if some of the gear gets wrecked. This pack has some well-thought-out features, but durability and comfort are not prioritized.
If you spend more time climbing rock than ice and snow, we'd recommend the Patagonia Linked. The REI Co-Op Flash 18 can also double as an ultralight summit pack.