I'm looking for a backpack for traditional, one-day, multi-pitch climbing.
Which is the best one? I'm inclined to the Marmot Ascent 40.
Got it. You want a clean, compact pack that's big enough to hold your gear but also pretty streamline.
You're definitely on the right track with the Marmot Ascent 40 ($129). It's a very clean pack--really just a bag with straps and a daisy chain for some gear attachments. But with 2,450-cubic-inch capacity, it holds quite a bit. Its full-length back zipper also makes it easy to find your stuff.
A very similar pack is Mountain Hardwear's SuperScrambler ($130). It's a bit more spacious, with 3,100 cubic inches of capacity. Same design—a clean bag with daisy chains, and a simple but effective suspension. I like its fabric, which is a silicon-treated lightweight ripstop that is highly water-resistant. The foam frame sheet can be pulled out and used as a seat pad in an emergency bivouac.
For something really spiffy, take a look at Arc'Teryx's Miura 30 ($199). A bit smaller than the Ascent, but it's a really well-organized pack, with side and back zipper and integrated hardware hangers for your gear. Two ways to tie on your rope, too. And like all Arc'Teryx products, it's beautifully made from top-notch materials.