So from what i heard, everyone will get 1 key from somewhere when they become available. Let's say i get a key for Eye of Azshara. Does my chest reward at the end of the week count the highest level i got my key to? Or the highest level mythic+ i cleared? Because let's be honest, not all dungeons are equally difficult, there will always be an easy one, and one of 2 things will happen:
1) You get the easy dungeon keystone and get phat loot/a lot of gold for inviting people to join.
2)You get the hardest dungeon and get screwed.

Either way, even if it's based on the best performance overall or your keystone performance, getting into a group for the easiest one seems to be the only smart thing to do.