Long story short; if I'm playing against someone with 0 MR items, is it better to prioritize pure AP items, or should I build Magic Penetration? I know that MP scales with AP. However, if someone only has their base MR wouldn't it optimal to maximize my AP instead of MP?
Also is their a soft/hard AP cap .... etc?
It all depends on team comps. If there tankiest champ has 60 mr rabadons deals more damage after 60 mr void staff deals more damage. Also was role was your ap.
mPen does not scale with AP or anything at all, unless you mean void staff, which gets stronger the more MR someone has.
Flat pen 
is best against low-MR targets, as you can even go into negative MR with enough, doing more damage than your base.
Percent pen 
is for targets with high MR, and it applies before flat pen. A target with 150 MR loses 60 to void staff and then 33 to sorc/liandry, leaving 47.
Pure AP is best when your champion has high AP ratios compared to their base damage and you are not dealing with a high MR target.
Each 100 MR/Armor acts like having +100% HP (100 is twice as survivable, 200 is three times, ect).
Note mPen is being moved around and changed in the upcoming rework of AP items.
