It is easy to assume a bigger bag means better photos. In practice, the gear that earns a permanent spot in mine is boring: two camera bodies so a failure never ends a shoot, one fast prime lens I know well enough to use without thinking, and a zoom for anything that changes distance quickly.
Beyond that: spare batteries and cards in a separate pocket from the ones currently in use, a small reflector that folds flat, and a plain grey card for quick, reliable white balance in mixed light. Nothing exotic.
The backpack itself matters more than people expect – I want to set a camera down mid-shoot and know exactly where it goes back, with padding thick enough that I do not think twice about it on uneven ground. Fewer choices in the bag means fewer decisions during the shoot itself.