The problem is that no one bag fits all requirements, and it never will. Every bag will be a compromise somewhere - too big for one need, too small for another. Sometimes you want a bag which is easy to work out of when you put it down. Sometimes you want a bag you can work from without putting it down at all. I've got over half a dozen bags of various designs and sizes and I wheel them out depending on my needs for the occasion.
Personally I dislike shoulder/messenger bags as they can swing about and get in the way - yet I have two of them and occasionally they are the best option.
Rucksacks (I have four which I use for photography) provide a much better spread of load when carrying plenty of kit, but for the most part do not provide easy access to gear without taking the bag off and putting it on the ground, which might be muddy or wet.
Sling bags (I have one) provide rapid access to gear without removing the bag, which is a huge benefit for speed, safety, security, but placing a load onto one shoulder is not always ideal.
Overall I give the win to my Lowepro Flipside 400 AW, which can take plenty of kit while not being overly heavy and bulky itself. With a rucksack design the load is spread well and the unusual rear entry to the main compartment adds security as well as allowing gear to be accessed without putting the bag down. The biggest negative is that despite ample interior space it is not ideally designed to hold two bodies at once, each with a mounted lens. Maybe I could rearrange dividers to provide that option, but I make do with one mounted lens and everything else separated for better overall flexibility.
To sum up the must haves for a general purpose bag...
- Lightweight in itself;
- Quick and convenient access to gear;
- Ability to hold two semi-pro (e.g. 5D3) bodies with lenses mounted (e.g. 24-70 and 70-200/2.8);
- Option for accessories like a flash, filters, food/drink, a tripod or monopod;
- Waterproof;
- Complies with carry-on luggage limits;
- Spreads load in ways suitable for the fully laden capacity;
- Ideally should not shout "EXPENSIVE CAMERA GEAR IN HERE".
Here are some of my bags -
http://www.talkphotography.co.uk/forums/showpost.php?p=4382205&postcount=15. I've since added another large camera rucksack and also use a lightweight, non-photographic rucksack when I just want something to carry a single camera/lens to and from a day out for walkabout shooting.
