Well for me its The Berghaus Munro 35l, The Snugpak Rocket Pack 40l, or a Karrimor Sabre 45l.
I have used the Munro for 15 years or so and its a great little sack, but i have just about worn it out. I always felt it was 5 lites (a set of billy cans) too small, so bought a Snugpak rocket pak 40l - Which is ideal sizewise + it has the option of fitting "Rocket Pack" or "PLCE" (Personal load carrying equipment) military side pockets to it, which can be detached and used on their own with a yoke, forming a really nice daybag. However I have never been entirely happy with the Rocket Pack, it seems over complicated, and the main fabric a bit to thin/soft. I can't quite put my finger on its a really good little pack, but just doesn't instill the confidence of the Munro.
So for me next on the list is the Karrimor Sabre 45l - getting a bit large for my liking, but less complicated and appears more robust. Someone has put together a special offer for us on them from an army surplus store, so they come in at about £40 with the optional PLCE/Rocket Pack side pockets. Only disadvantage asfar as i can see is that they are in camo, not my first choice.
Thats my 2ps worth
Cheers
Rich