WARNING: IF YOU DO NOT PACK LIGHT, THIS BAG IS NOT FOR YOU!!!
Ok, that being said, let's talk about the best bag on the market... I've used this Hyperlite bag for years. From overnight trips to massive traverses, the 55L is all I need. I've put over a thousands of miles on it and it still looks pretty damn new. No rips, tears, cuts, loose threads...nothing. The kangaroo pockets are excellent for gear you need to access throughout the day. The compartment has plenty of room and even a net inside I use for a camelbak bladder with a small hole to allow the hose to come out and rest on my shoulder strap. As you continue throughout your journey and you have less food or you're only going on a short trip and don't fill the bag, the rolltop feature and compression straps allow this bag to shrink down so your stuff isn't jostling around. Really allows for versatility. To top it off, it's extremely water resistant (almost waterproof) so I never have to use a rain cover or internal drybags for my gear, unless I'm canyoneering.
If you're unsure if you're packing light or not, here's an idea of the gear I'm bringing:
Big Agnes Flume 30 - 1lb 6oz
Big Agnes AXL Air - 10.7oz
Big Agnes Fly Creek HV1 Carbon Tent - 1lb
Jetboil MicroMo - 12oz
Sawyer 16oz Water Filter - 2.5oz
Petzl E-Lite Headlamp - .9oz
SOG Reactor Multi-Tool - 4oz
= 4.25lbs in gear.
Now add lightweight food, gas, minimal clothing, and a small flask of whiskey (because duh, you need that) and we're not talking more than 10-15lbs total, and that's for multi-day. I did the Sierra High Route with roughly that using this pack and it was awesome. Even more hardcore ultralight packers can go lighter by using a tarp (like the one Hyperlite makes) and a foam pad, but I'm pretty damn stoked on how light my setup is and it's also comfortable.
This pack can obviously carry more weight than that, but realize it doesn't have a stiff frame or waist strap (which is why it's so light) so buyer beware if you have big heavy gear.
So if you're wanting to round out your lightweight setup, this is the pack!