No bag is the "best" overall, it depends on your preferences and needs. For a compact backpack that can carry quite a lot, I recommend the LowePro Mini Trekker AW.
I have one myself, and I carry quite a lot inside it: 450D + battery grip + Tamron 17-50 attached, 70-200 f4L, Sigma 30 1.4, Sigma 10-20, Sigma 90mm macro, 430EX + Omnibounce + Flip-It Diffuser, 77mm CPL, Giottos Rocket Blower, 2 extra sets of AA batteries, extra memory cards, other assorted bits that I can't remember at the moment... that's not counting the front pocket, which can expand quite a bit and carry a sandwich + a small bottle of water. There are also SlipLock attachment points which allow you to securely attach LowePro's line of accessory packs (extra lens cases, water bottle holders, pouches etc).
It's surprising how much gear you can actually fit into that little backpack, including a full-size tripod (comes with a tripod holder built in!). It's really small, too - I was surprised at its tiny size at first.
If you need bigger, there are lots more choices, but none that really match the MiniTrekker for size. The harness system is quite good, too. Comfortable for long(ish) periods.
EDIT: Forgot the product link. http://products.lowepro.com/product/...AW,1965,14.htm