Very much a personal taste, but mine is Berghaus Freeflow 35 + 8. That's fits waterproofs, insulated jacket, windproof jacket, lunch, and in one pocket all the bits, headtorch, batteries, gloves, beanie. Top pocket for compass, etc.
Also can depend on whether you want a bladder. Some like these so you can drink on the move, my preference is to stop for a drink
But by far my main requirement is a rucksack with side pockets, many of the current ones seem to have gone for those half size mesh pockets, which for me is asking to lose whatever goes in it, much prefer the side pockets, top pockets etc to all have full zips. This has meant to resorting to eBay given the manufacturers seem to have gone awa y from these now.
Just something to think about