Was really bored on electronics class, so i started calculating random lol stuff... meh... Seems like the most effective stats are AP and HP right now. After these two are AD and MR.

Ability Power


AP can be easily stacked, eventhough most skills don't give out full AP ratio, on average i'd say it still scales really fast. Also there are lot of AP items that give high AP and tankyness both. Even if we look at 0.6 ratio, APCs can reach 500+ AP without any problem, which is still 300 magic damage. I would also note here that MR is harder to stack compared to armor so 300 magic > 300 physical.

Health


HP is probably the best defense stat atm. A Warmog's Armor can give you insane boost, and there are lot of other items. Building HP makes you harder to kill, but what about armor and MR? Well they make your HP more effective, if you take 23% less damage, your HP will last 23% longer, pretty simple. The thing is, early it's easier to double your HP than increasing your armor or MR that it will have the same effect, also it goes only vs one of the damage types, unlike HP. And a little thing called true damage... only HP protects you.

Attack Damage


AD is still really strong despite anything above. Why it's so strong? It boosts auto attacks aswell as AD based skills and there are lot of AD items granting other useful stats. Boosting auto attacks just totally makes up for the harder stacking. The only problem is that building armor is relatively easy and enemies are forced to do if there are AD based threat, but after a while you just can't build enough armor.

Magic Resist


MR is really effective, becouse AP ratios are so sick, even building a small amount of MR greatly reduces the incoming damage. Since defenses are percentage based, the more incoming damage, the more effective is the reduction, pretty simple. However it's harder to stack.


So i came to a few conclusions, probably some other did too... anyway...
flat AD reds and quints seem to be more effective than flat ArP ones, early game they are just more effective, late game it just doesnt make difference. Only difference if you shoot on low armor with really high AD, but those targets usually die pretty fast anyway, and still not too much difference for late game, not that much you should care about it.

Warmog's Armor is insanely strong early game, better than any armor or mr stuff... Even really strong later, but getting an early one and stacking armor or MR later is probably better. Frozen Mallet pretty much the same but with slow and AD instead of HP regen.

Sunfire Aegis is another insanely strong item. Gives nice amount of HP and armor with it, for a low cost (in my opinion). It's actually barely weaker than Randuin's Omen but with better price. Also the passive 40 magic damage ticking every second. Noted above that MR is harder to stack than armor so: sunfire cape passive > 40 AD. Not to mention it's AOE and ticking 1s means it's faster on some champs than their auto attack, atleast for a while, but even after that it's still pretty close to it.

Black Cleaver gives AD, gives HP, gives CDR... pretty much everything you'll need. Also rips off the armor that ppl hardly stack to be enough to survive. This thing is evil. Not sure which is more evil, this or sword of the ruined king with it's HP tearing.


Gotta look at AP items more... i'm sure i'll find some really effective ones.