I have 3 different Tamrac packs I use, but for backpack style I never get into using them. I like the messenger style bags as it gives me instant access without setting it down.
I have a LowePro large backpack to throw things in for a shoot setup on location, but I will never buy another without wheels. Rolling backpack is the only way i would get one.
Even on hikes I use a messenger and sling it back. Vanguard Velocity 38 is darn good one. I have the original from years back, and the only thing that came off of it is the clasp cover which makes no difference.
But I never understood how a backpack works for street, or on the go type of shooting. Maybe long hikes that have a end point to stop set things down and shoot...but I dont do those without a bunch of in between shoots.
I think CaseLogic, who surprisingly is making some interesting packs, has a "backpack" with top access to camera. That, if bigger, might be worth a look.
I have about 15 to 20 different camera bags/packs, and I still go for the large messenger Vanguard OR the rolling Tamrac CyberPro Express....
https://www.bhphotovideo.com/c/product/321683-REG/Tamrac_526301_5263_CyberPro_Express_Briefcase.htmlThe Tamrac 5797 maybe a good backpack...
http://cheesycam.com/best-dslr-video-hybrid-backpack-roller-bag/