I use a variety of bags and packs, but two that I highly recommend you look at are the Vertex bags from LowePro (which come in three sizes) and the Chest Vest. While the Chest Vest isn't a backpack, it's a very ingenious and well-balanced harness system in which you can securely carry a lot of gear. I use it more often than any of my backpacks. Good luck.
Canon 40D, 24-105 f/4, 500 f/4, 100-400 f/4-5.6, 10-22 f/3.5, 100 f/2.8 Macro