You are going to want a pack with some type of suspension system that transfers the weight to the hip belt via some combination of metal stays and/or plastic frame sheet. Completely frameless packs like the Wild Things Andinista, Cold World Chaos, and Arc'Teryx FL series packs lack this feature and are therefore uncomfortable carrying loads heavier than 35 pounds. I have used the WT Andinista, ArcTeryx FL 45, and CCW Chaos packs. The argument is that one cannot climb hard alpine routes with more than 35 pounds on the back, which I agree with, but most climbing objective require some length of approach during which far more than 35 pounds needs to be carried. My recommendations, therefore, include packs that have some type of hard suspension that transfers weight off the shoulders onto the hip belt.
CRUX AX-50 or AK-47.
https://www.crux.uk.com/product/ax50 These are both light, tough, and purpose built for alpine climbing. They use two vertical titanium stays to support loads. FWIW, I own the AX-50 and I find it small for '50 liter' pack, maybe even smaller than my trusty BD Shadow 45 L, but incredibly light. My two gripes are lack of a crampon patch and smallish top lid pocket.
CiloGear Worksacks
http://www.cilogear.com/45lws.html use a plastic framesheet and aluminum stay to provide support. I have used these packs and like them for alpine climbing, they carry and climb very well. I do find the design to be inherently less durable then other packs and have never seen such beaten down, torn packs. They are designed to be configurable and one can strip everything down, including the top lid, frame, hip belt, and the straps which leads to accidentally lost straps. That said, if you don't mind replacing your ~ $300 pack every couple of years, they are a good choice.
McHale packs.
http://www.mchalepacks.com/ultralight/index.htm These are custom made packs so the fit is incredible and you can have it built as simple as you want. McHale packs use two aluminum vertical stays to transfer weight to the hips. Most of the photos of their packs on their (terrible) website have extra pockets and zippers and what not. I have a SARC that I had Dan build with a small hip belt, no zippers or pockets on the body, and the lightest materials available at the time. The carrying ability/comfort and climbing ability are outstanding, as is the durability. You will not find a more durable pack. My SARC pack is now 20 years old and has been dragged all over North America, including three Alaska Range trips and still looks brand new. A couple of suggestions should you decide on a McHale; order your pack with a small, single buckle hip belt ("better than an Andinista or some such shit" in Dan's words), no zippers or pockets in the pack body, and made of Dyneema grid fabric for a light weight, stream lined pack. Additionally, \have the pack built with out a top flap, or tabs and handles on the top lid which allow it to be configured as a fanny pack. Useless extra weight.
A couple of other packs that I really like are older, used ArcTeryx Nozones or the Gregory Alpinisto 50.