I think that the perfect camera bag is not a camera bag. That’s the first criterion. If you accept this, read on, if not, then keep carrying around anything that says “I’m a photographer.”
My three favorite messenger bags for wearing while shooting and/or traveling are a black Patagonia Half Mass, gray Timbuk2 Mustang, and a Filson Medium Field Bag. The Half Mass has had a design change recently that gives it protective corners on the top hinge point so that when the flap is down, the top corners cover the opening and keep water out. It has a grab handle and one bottle carrier.
The Timbuk2 Mustang (or some other horse name) is completely coated with waterproof material, and the top of sides connects directly to the flap, and the corners fold to block water from getting into the bag. It is the most waterproof (not that waterproof is relative) but has no grab handle or external bottle carriers This bag may be recently discontinued. Look for it on ebay and in bicycle shops.
The Filson Medium Field Bag is waxed canvas the same weight as blue jean material. It is the lightest weight and closes up well against rain. It has both grab handle and bottle carriers, albeit maybe only big enough for a beer can.
For the two bigger bags (Patagonia and Timbuk2) a standard size Billingham insert will hold and protect a camera and a flash. For the Filson, one of the Domke inserts works well to hold the camera in place. Use another insert for flash or extra lens.
The Patagonia comes with a hideously noticeable reflector strip all the way across the front. You can use duct tape to cover it.