It's tough to give a good answer to this without more clarification...
How long would your average trip be?
What kind of gear are you planning on carrying?
The size of the pack you'll need/want depends in part to the answer to those questions.
Personally, I prefer internal-frame packs: I have had great luck with Vaude and REI packs in this category. The Vaude Assymetric 50 is the pack I'm using currently (can be adjusted to fit anyone adult-sized, down to a 12 year old), and I had the REI Morningstar 65 several years ago (quite a bit more cubic inches than the Vaude pack).
From other's recommendations and good experiences, I would also definitely include for future consideration the following brands: Northface, Kelty, and Lowe Alpine.
You will find different opinions on whether an external-frame or internal-frame is better - but for me it comes down to personal preference. Whichever one you pick out, do everything you can to try one on before you buy it (or at least, before you decide not to return it)!
I've found that internal frame packs are better at wrapping around your body - and they've given me a more 'ergonomic' fit, and they don't move around as much for me while on the trail. External frame packs that I've used have been better at ventilation, and being more flexible as far as load-balancing.
Good luck, and happy hiking!
