There is no best car, hence there is no best backpack.
As a rule of thumb: It is easy to buy a backpack that is too big. It is nearly impossible to buy one that is too small. The experience of a traveler is in an anti-proportional relationship with the size of his or her backpack.
This being said, the best back pack I ever had was from Burton. I and I am still heart-broken that it got stolen. It is not produced anymore.