Season 6 Runes Pages
These are my Season 6 Runes,
I can explain why I run these runes,
Tanks: The Reason why I have soo much armor is because there is too much armor pen items currently in the game. Magic resist items are less costly than they were in other season which means easier to afford. One magic Item can make a huge difference, because they always have a built in passive that let you sustain or higher health regen.
Mages: Magic Pen is too important in this game, there is soo much item that has Magic resist in it. 15 Abilities power will do less damage to someone who has 
than someone who build pure Magic pen. CDR is easier to get, which helps you spam more spells, but the Abilities power has been soo dramatically reduce until you get Rabadon who cost a shit tons of golds. In others words, you'll do Fixed amount of Damage which mean it cannot be reduce by MR or it cannot increase itself. It all depends on your Base Damage and champion Abilities Ratio aswell as Cooldown.
AD champions:
Do I need an explaination? 
.
There is too much, armor pen in the game. All you need to do is get your Attack Damage as High as possible.
There is always an AD item that offers both sustain and defense aswell as armor pens.
. When your opponent is stacking armor/mres, that's when you want the % pen. So I don't understant your statement that you want the flat magic pen BECAUSE your opponents will stack Magic resist. Or do you mean that their carries will buy one magic resist item so you can get more than 30 pen and get full value out of it?