Keeping your camera gear safe and protected, yet accessible, particularly when out and about, invokes a question the answer to which involves the length of a piece of string. There is no "best" answer, as it is down to personal style, preferences, situation, climate, mode of transport, type and volume of equipment, and whether it is going to rain or not.
Here's some ideas based on how I carry my gear.
Between my wife and I we have three DSLRs, about nine lenses, three flashes, triggers, batteries, many memory cards, four tripods, a monopod, a variety of filters, macro tubes, cleaning kit, and range of other related knicknacks. Of course I don't carry the whole caboodle around with me wherever I go - I only take what I think I am likely to use.
(What to pack for a particular scenario is another length of string which we might explore in the future. If you are going on holiday and want to know what to pack, please ask me in the Comments below, or use the Contact button. I'd be happy to share some ideas with you.)
For the past eight years I've been using a LowePro Primus AW backpack. It has travelled extensively across three continents with me. This is a rugged, all weather, medium sized bag. I like that the main compartments open from the front. Most backpacks open from the top or back allowing villains to fiddle with your stuff. With the zips against my back my stuff is safe from prying hands.
The pack has two main, equally sized compartments. The upper is less padded and I carry a rain jacket, sandwiches, hipflask, binoculars, cables, harddrives and pretty much anything else I might need. The lower compartment has padded dividers which can be configured for any camera/lens combinations. The lower compartment also has a zipped opening on the side of the bag, theoretically allowing one to extract a camera without removing the backpack. In practice this is not a comfortable operation.
For the all-weatherness, the Primus has its own raincoat tucked away in a secret compartment. Many other camera bags have this feature too, and it's worth looking out for.
Typically I would use this bag for outdoor or travel adventures because it holds a mix of camera gear and real-life stuff. Like my lunch. When I'm going on a specific photo job where I might need flashes and a range of lenses at my disposal, I'm likely to take my LowePro Nova 200 AW. This is rectangular, dedicated photographers shoulder bag with a bunch of padded inserts. It's not massive, but it holds the flashes, a couple of lenses and all the ancillaries securely. I wouldn't walk too far with this bag as, when loaded, it can be quite a strain on the shoulder. At home, it is the perfect container to keep some of the less-used gear stored neatly.
On a recent trip, I needed to carry quite a bit of gear on six international and domestic flight, including my 150-500mm lens. It would have probably have fitted into the Primus backpack, but then I also wanted something less bulky and conspicuous to stroll around the cities with. With a bit of research I found that airline carry-on luggage is more-or-less quite well defined, and the airlines seem to be working towards a standard size of bag that can fit in the overhead bins. http://www.skyscanner.net/news/cabin-luggage-guide-hand-baggage-sizes-and-weight-restrictions
To maximize my carrying potential, again with a degree of research, I found the CabinMax series of luggage. Their "Metz" bag is 44l and measures 55x40x20cm, which is exactly within almost all airline limits. And bring a soft bag, it can be squished smaller if necessary. It is also very light and quite inexpensive. See it here on amazon.co.uk: http://amzn.to/1RR31jU
Then, to protect the camera and lenses, I used a padded inner, made by Caden. It measures 29x14x22cm. Which lens is attached to the camera determines how the dividers are configured. http://amzn.to/1RR3aDP
Here's the clever part. I also picked up a little day-pack from The Friendly Swede that folds up into itself so that I could have an unobtrusive means of carrying a camera, rainjacket, sandwiches, water, etc around town. The Caden padded inner also fits perfectly inside, so it becomes a decent camera bag, that doesn't look like a camera bag. When its rolled up it takes up less space than a scrunched up t-shirt. http://amzn.to/1RR3fHH
These three items together come it at about one-third the price of a Thinktank Airport Commuter backpack, and arguably considerably more adaptable for different environments and situations. http://amzn.to/1q93y4O (And this is one of the cheaper "real" camera bags.)
By the way, I also have a set of luggage organisers. Very lightweight zipped bags for inside my main luggage to keep shirts, underwear, socks, laundry separate. I use one of these for all the photography odds-and-ends too. http://amzn.to/1RR3ti8
That's enough about bags. The bottom line is that there is no one bag for every situation. Some women like to collect shoes. Photographers collect bags!
For walking and shooting, you need your camera close to hand. Tourists and new camera owners think that it's cool and proper to use the strap that came in the box and advertises the brand in big letters. Please don't. There are a few reasons why not, the chief being that it is not comfortable or good for your neck.
Consider getting the BlackRapid style of sling strap to carry your camera rather. These straps go across the body, from (usually) your left shoulder to right hip so you camera hangs ready, like a gunslinger. (Ladies, BlackRapid do a version that is supposedly kinder to the boobies.) The winner for these straps is that the clip that attaches to the camera is able to move up and down the strap freely, so the strap itself stays comfortably fixed on your shoulder. http://amzn.to/1q942Yw
As you would expect, I'm a frugal miser. I have the BlackRapid Sport but when I needed a double strap to be able to hang two cameras for a wedding I was shooting, I opted for the knock-off brand at about one-tenth of the price. Some of the padding did start to fray after 18 months of use and the metal bits weren't as slick as the BlackRapid's, but the cheap version was definitely good value for money. It can also be split and each side used singly. http://amzn.to/1RR3IcW
Peak Design has a nice looking strap called the Slide. I touched one at the Photography Show recently. It certainly seems to be a quality product, but I personally don't think it is for me, mainly because the whole strap slides, unlike the BlackRapid design. http://amzn.to/1RR40R8
I have Peak Design's Capture Pro. This little gadget will attach to any strap or belt, and has a quick release mechanism for the camera. Sometimes I will use it on the left shoulder strap of my backpack when walking about, keeping the camera easily accessible. Scroll down an watch their video on their website. And buy one here: http://amzn.to/1RR4gQa
Some folks like to use a hand strap on their cameras. I've tried one once, but it wasn't for me at the time. Hand straps connect to the right side of the camera and go around the back of your hand. You can release your grip with your fingers, and have the camera not drop to the ground. There are many on the market, and although I have not tried it myself, I would recommend the Peak Design Clutch because it seems to be a well though-through, quality product. http://amzn.to/1q965vM
There is quite a lot of information here. The takeaways should be that you don't need to have a bag specifically designed for cameras. A "normal" bag will reduce noticability from the villains. Your bag needs will change over time and by situation so be prepared to have a different bag for every day of the week. Save your neck and don't use the strap that came with your camera. Rather get a slider strap like the BlackRapid. Think about separate storage for ALL your gear when you're at home, and don't forget about storage for the bits-n-bobs - batteries, cards, cloths, raingear, hipflask, etc - that you will need to carry around with you too.