…sadly does not exist. There are a number of different reasons for this. First and foremost, people have different preferences or expectations. Some may prefer the rapid access offered by a sling or messenger-style bag, while others appreciate the comfort of a conventional backpack. Photographers with many different lenses may want to ensure their equipment is protected against any potential danger, while others may choose ease-of-use over security.
As so often, planning is the key. For a quick weekend getaway, a messenger-style bag with a pouch for a camera with a travel lens and room for a water bottle may be perfect. However, after a few days such bags can begin to cause discomfort because of the uneven weight distribution, and your top of the range new, small mirrorless begins to feel quite a bit like your old full-frame DSLR.
On the other hand, taking your fantastic backpack with top-level protection may seem like a good idea at first, but after constantly banging into people on crowded public transport for days you may be looking for an alternative. In addition, such backpacks are often so perfectly designed for camera equipment that they offer little flexibility. Suddenly, clutching a bag of souvenirs or food makes it even more difficult to extract the camera.
If you know that your trip will involve a little bit of everything – sightseeing, shopping, walking etc. – it may pay to invest in the smallest possible camera bag you can get away with, then throw it in a backpack. This will allow you carry a raincoat or sweatshirt as well as whatever you may buy but will ensure that the camera remains safe.
In short, consider what the primary purpose of your bag will be in advance and make a decision based on that. Otherwise you may find yourself preferring to leave the camera behind rather than suffer any more shoulder pain or dirty looks from people in crowded spaces.