I haven't gone through loads of bags so can't say what is the best bag around but of the several I've used, they've all fitted a criteria.
I have a Kata DR465, which is my day pack that I take out walking with just a body and lens. The bottom camera section easily fits a D2x with 17-55mm plus a flash or a small lens. The top section carries food and accessories nicely and it's a very, very comfortable pack to wear all day. The shower cover is handy too. I use this as my day pack when I'm snowboarding, with a compact and some food in there.
For everyday use I have a Vertex 200 to hold the following:
*2x Nikon D2x bodies
*Nikon 35mm f/1.8
*Sigma 14mm f/2.8
*Nikon 60mm macro
*Nikon 17-55mm
*Nikon 70-200mm VR
It holds all that without any issues and if I bothered to configure the dividers a bit better, then I could get two flashes or another lens in there for sure.
It's well padded, which helps when it's being thrown around, in and out of car boots and onto planes and boats, and the rain cover is a godsend, although it's pretty much waterproof anyway thanks to the heavy-duty material it's made from. The laptop compartment is good, although it's a dumping ground for CF cards, chocolate bar wrappers and random junk. The front pockets are good, especially the memory card pockets.
It's a comfortable bag to carry thanks to very adjustable straps and the front bracing straps help support the bag when you have a big hike.
Probably my biggest gripe about it as a travelling rucksack is the lack of rear opening, as it's easy (when the rain cover isn't on) for someone to quickly unzip a pocket and nick something. May move up to the 300 version at some point soon.
Burton Zoom 25L - I bought this for taking my gear out on the slopes and it carries pretty much everything the Vertex does, just in a less padded, lighter format. It's very comfortable to wear and although not good for torrential rain, it handles snow fine and light showers. The fleece-lined battery pouch is a nice touch, as is the rear entry so you don't get snow inside the bag when you access the gear. Loads of storage, more than the Vertex, and the shovel pocket doubles up nicely for carrying clothing and the vertical boards carry straps work well for strapping on a tripod. Just a great bag and one I'll keep.
2x Lowepro Nova 4 bags, which are great for carrying a body and a lens (and maybe a flash), although one is used specifically to carry flashguns, triggers, batteries and lighting accessories like coldshoes and ballheads. The other is used as a hard drive caddy, carrying four Lacie Neil Poulton drives, all the cables, plus a mouse (for the laptop), a spare MBP charger and other cables for phones and connecting cameras.