Well, backpacks are like airplanes. How big do you need it and how much do you want to spend?
Serioously, though I assume that you want a big day pack that will also do light overnights? My favorite old-school pack in that category is the Mountainsmith Bugaboo, about 2200 cu in, narrow vertical style, also great for skiing. If you can pick up one used, I recommend it. I still have mine that I purchased in 1991, will never sell it. Unfortunately, Mountainsmith was never the same after Patrick Smith left the company about 10 years ago, and the new packs aren't the same.
In terms of new packs, I like the Gregory Z35. It's available in three sizes, 2000 to 2300 cu in. Great suspension, nice vented panel to keep the pack away from your back for cooling. Lifetime warranty.
The best thing is, of course, to try it on with some weight in it and buy what fits
you. I highly suggest you purchase from a retail store that allows you to try on the pack and return it if you don't like it. REI does this, will also do mail order, and you can become a member / owner and get about 8% of your purchases back at the end of the year. I've seen people return items to REI after 3-4 months, no problem.
I would not suggest any of the camelbak style packs for anything more then short hiking - they lack any sort of suspension and aren't comfortable carrying more than about 10 lb. You're better off adding a water bladder (any brand) to a good pack.
TODR