I've spent a long time looking at back packs recently but with a specific requirement.
I'm going on Safari and want the ability to carry two bodies, one with a 100-400 mounted, the other with an 18-200 mounted, both need to be quick access as I want to keep them in the bag when not shooting. I then want the ability to carry my macbook and a few other bits.
In the end I got it down to two, the Lowepro Pro Tactic and the Manfrotto Redbee. The Lowepro is very expensive but very good, it has the ability that you can attach almost anything you want to it because of the straps that go around it, it's really well built, lots of padding and really comfortable, but it's £250.
The Manfrotto is £99 and at the moment you get a free Pixi Evo tripod, so in effect the bag is £55, it's still really well made, looks nice, can hold everything I need and has the side access and top access. The only bit that lets it down is the lack of pockets on the outside for things like SD cards or a water bottle etc.. but it's £200 cheaper! So instead I've bought an SD card wallet I can reach for through the side pocket and a separate water bottle carrier that I can clip to the handle of the bag.
https://www.manfrotto.co.uk/freepixi
The best thing you can do really is go to your local shop, such as LCE and see what they have, there's a lot of choice out there and seeing them in the flesh is so much better than someone elses review.
LCE even price matched Amazon on my Manfrotto bag!