Obviously the camera bag requirements of photographers are as varied as their respective artistic visions. Therefore there is no perfect camera bag that does everything. Some bags are luggage designed to protect gear on airline flights and some bags are simply a way to carry a camera and a few lenses. The bottom line is, you simply have to figure out how much “stuff” you need and how to carry it efficiently.
Which brings up my personal choice of camera gear. While I have done jobs where I had to fly to the location and bring multiple camera bodies, tripods, a tethered laptop, lights, stands and specialty lenses like tilt-shifts and macros. This array of gear required a combination of hard and soft cases for the sake of protection. But my primary photographic mission is documentary and environmental portraiture of entertainers. For me the ideal is a discreet backpack that holds a full-frame mirrorless camera and trio of fast prime lenses (typically a wide, normal and short telephoto). Plus a few small camera accessories and personal items.
This concept of carrying a single camera body and a trio of lenses is nothing new and goes back to the days of the father of modern photo-journalism Henri Cartier-Bresson. While the vast majority of his Leica captured images were taken with the Zeiss 50mm f/1.5 Sonnar lens, he usually carried a 35mm and a 90mm prime (just in case it was needed) in a small shoulder bag along with rolls of film. I know a lot of DSLR shooters that carry three lenses such as a 24-70mm, a 70-200mm and a fast (50mm or 85mm) prime for low light, portraits and or extreme shallow depth of field use or perhaps a macro. This three lens approach seems to be a logical method of being prepared for a wide variety of subjects, with a minimum of gear.
On another note, if a candid image is what you are after, it pays to be invisible to the subject. In the streets of Paris, the much admired Cartier-Bresson often wore a suit and tie, along with the small shoulder bag for his gear, which gave him the appearance of an ordinary businessman on his way to lunch. This stealthy approach enabled him to come in very close to the subject, without them being aware that he was about to take a photo. Thus avoiding the self-conscious gaze of a posed expression, a tell-tale sign that too often lends a death knell to the street photographer or a person doing documentary work. Of course in other fields of photography like: sports, landscape, architecture, wildlife, macro, weddings, formal portraiture and fashion, such discretion is a non-issue. Lending to itself, the copious use of the roller board suitcase, ATA case, giant backpack, photo vest, oversized fanny pack or belt mounted lens pouches.