Where should you position the heaviest gear in the backpack

Depends on what you're worried about:
Yourself
Agility/Movement Speed
The Lighter Stuff In your pack

Really, the general rule: Pack the heaviest items at the bottom, keep what you're more likely to need at the top. Though always be prepared to NOT have your backpack and still be ok for a little, so it's best to make sure you have some extra stuff that is important in any pockets you have, preferably not the back pockets. 

But for your backpack, just keep the heavy stuff at the bottom, so not to crush any lighter objects, and keep the most important objects you are more likely to need sooner at the top.