Like everyone, I've gone around with different bags. In the end, I have come to really appreciate the waist-belt with shoulder strap style. When I'm shooting, I'm often squatting down or climbing over things, and I really want the security of a well-attached bag.
The best size for me has two dividers and can hold 3 prime lenses and a camera (with lens). When I'm traveling everything is in the bag; when I'm shooting, the camera is over my shoulder and the lenses are separated by the dividers.
I have two bags that work well for this: the smallest Think-tank, the "Speed-Demon", and an old Lowepro "Photo-Runner" (wish I could find these on Craig's list). The Think-tank is well-made, well-designed, slightly heavy, but very comfortable. The Lowepro is lighter, less bulky, and completely satisfactory.
Either of those bags works perfectly for my Leica or my Pentax K5 kits. My Pentax lenses are all FF (unrequited longing for a FF Pentax DSLR), and have no problem fitting, and that includes (barely) a 300mm f/4.
My solution feels good because it is small, efficient and secure. If I was carrying a pro-level Canikon or any zoom lenses above the consumer grade, I'd need a bigger bag.