I have many photo bags for different purposes, but my all time favorite has been Lowepro Flipside Sport 15L AW. I have used it billion years, hiked billion miles and nothing is broken or completely worn out. It’s very light and has one of the best tripod attachment system I have ever used. It has a rain cover that comes out from the special compartment at the bottom of the bag and is permanently connected to the bag. It has flipside access that I find to be best solution (at least for me). So unless it will brake down, I keep using this amazing bag.
Unfortunately it’s not possible to buy this bag new anymore, but Lowepro has many other amazing bags. The most comparable series are Flipside and Photo Sport series.