Like many (most?) of you, I've been through many bags on the quest for the perfect one. I mostly used a Think Tank Turnstyle 20 sling which allows easy access to the gear once spun around to my front, but some times I need more than it'll hold. I have a 5DIII and mostly L zooms, including the 70-200 f/2.8 II so they can be bulky.
I have a Think Tank Retro 30 and while its a well-built and comfortable bag, it has some usability issues. Being a messenger bag, the flap gets in the way. It takes two hands to push the bag away from my body and tuck the flap out of the way before I can access its contents. The bag is very soft and conforms to my body, which is good and contributes to the comfort, but this also means that it wants to collapse while I'm trying to stick a lens in it. This isn't such a problem while the bag it sitting on a hard surface, but its a major issue while I'm wearing it. I prefer to wear a bag across my body and can't always set it down to use it.
I've looked at the Tenba DNA series which has a zipper on top but I don't know if it'll help. I think the issue is more about the stiffness of the padding, especially dividers, needed to keep the lens slots open and easy to access. What bags should I look at?