In reply to ROSP:
Unfortunately no one pack does it all as 'alpine climbing' covers such a wide range of possible routes / conditions and preferences. I have an old Macpac rucksack which was perfect for the New Zealand Alps - tough as old boots, can be dragged through steep rain forest, floated across rivers, scraped against sharp rocks, swallow gear for multi day approaches and routes, even survive being attacked by Keas but rather over the top for the European alps. Originally I used to have one rucksack to do everything so it was a major compromise a lot of the time. Nothing wrong with that though, I'm lucky enough to be able to afford to compromise less now.
In my opinion many rucksacks supposedly designed for alpine climbing are over featured, too heavy and in some cases not robust enough to last one pitch of a Chamonix chimney. Now I have managed to collect three different rucksack types which reduce the compromise somewhat:
1) A 25 litre very lightweight pack for when I am not carrying much. This can be alpine rock routes including carrying the pack whilst climbing or fast / light day routes or routes from a hut. This is the sack I use the most often. Personally I currently have a Patagonia Ascensionist but there are plenty of great packs out there.
Features I look for here are very light, very simple, can take a full rack ( if needed ) and rope as long as I'm not going too far with the load, takes two axes. Simple small waist belt, stays stable and out of the way when I'm climbing. Pack can hold my boots, crampons, belay jacket food and water during the climb ( assuming a rock route in rock shoes ).
2) A 35-40 litre pack for routes which may require more technical gear or an overnight bivvy. This is a tougher pack, more comfortable with loads especially if there is a long walk in / out. I currently have an old Berghaus Arete ( I cut away some of the unnecessary features ) that is nearing the end of its life. Looking forward to getting a better replacement.
Features I look for here are toughness, compact load carrier but reasonably light, ideally a floating lid, simple waist belt, somewhere I can access water whilst on the move , ability to take axes.
3) A 45-50 litre pack for multiday epics that is an excellent load carrier but ideally strippable into a lighter sack for climbing. I currently use an old school Pod Black Ice but after many years of service it needs replacing.
Features I look for would include being a tough and excellent load carrier, including able to carry skis, that ideally can be stripped right down. A floating and removable lid, removable back support, larger waist belt removable with a thin basic one to take its place.
The most important features in any rucksack in my opinion is that it fits you comfortably ( different makes and models suit different people ) and carries the load you intend to carry.
If I had to compromise again I would probably go with the 45-50 litre pack as I can do all the climbing I want to do with that but I certainly couldn't with a 25 litre pack. If not doing extended routes / trips / bivvies / camping then a 30-40 litre pack would suit - that size would be fine for a day or two of bivvying assuming the rest of your gear is compact and light.
So to answer your question; I don't think there is a perfect or best alpine rucksack because it will depend on the nature of the climbing, the size and weight of the gear you are carrying and the personal fit for the climber.