I love my Craghoppers Dri-pac 50. I picked it up at a sale in Portsmouth some years ago on a whim after having done extentive research and having come up with nothing to really suit my needs. I don't know if they are still available. It is big enough to take what I really need for a week-long trip but not big enough so that unnecessary luxuries can creep in. It has a separatable fold down drybag which is a godsend and a dynamic strap system which makes it very comfortable to carry. Is has an adjustable back system which I had to experiment with to get right. The only downside is its weight empty, but it feels light enough loaded and makes my usual load of 12-15 kilos easy. A few weeks ago I did a three-day trek with it with no problems.
There are some good suggestions on previous posts, but I'd warn against getting anything too big as there is always the temptation to put in extras. However, I stuggled for years with a climbing pack which was too small and not really suitable for walking long distances. As someone above has said, make sure the pack fits you, Research on line, check out your shortlist by visiting a good shop and trying them on. Take your time, you'll be carrying it a lot longer and you'll grow to love it or hate it. If you don't get it right first time you'll end up buying again!