Itemization is terrible Riot and you need to deal with it now
AD is better than AP at all stages of the game. It used to be AP was early game and AD was late game and kits made the difference in where everyone fit in. Now if you have AD there is no difference you are relevant all game. There is way too much defense and damage on the AD items now.
120 AP + 35% AP. 3800g.
65 AD + 10% CDR + mana regen passive + up to 20% CDR from other sources. 3600g. In order to even beat this as death cap you need to have 3 items to their 2.
75 AD, 10% CDR, 15% healing on all physical damage. 3500g. (this item shouldn't really even exist, there's already 4 lifesteal items and it is 100% better than 
in most situations). This item alone shows the power creep that they gave AD items. vs. is just too damn obvious.
100 AP, 45 armor, active remove from play. 3500g.
vs.
55 AD, 10 Armor pen, 40 MR, automatic passive that gives a shield and adds heavy sustain. 3250g.
Maw is obviously better by being cheaper and giving much better and more stats while also having a more reliable effect.
AD is supposed to be theoretically around 60% of AP based on how all the mages were balanced and they are supposed to be more expensive for their slots. But when we compare individual items AD still over powers AP for less gold efficiency. Right now its cheaper to build AD for about the same ratio of stats and the AD items tend to have other effects attached to them.
Note: 
= . But > in gold efficiency while being cheaper. Yet another example of power creep.
If this isn't dealt with in the next 2 patches or AP itemization gets any worse this game will just dig itself even deeper into the terrible state its in. By going AP you are gimping yourself right now. You are better off playing an AP tank over playing AP right now.
