Nice tier list. Please take the time to go to this link

http://www.freeriotpts.com/?ref=3prB4GSKd

It's really hard to argue for which item is better without first asking: Who are we planning on building it on? For instance it's been pretty decisively shown that Infinity Edge is better than The Bloodthirster on AD carries, to the point that building the later before the former seems blatantly wrong, but on someone like Lee Sin, where crit chance isn't as valuable, and lifesteal is more valuable, The Bloodthirster becomes the better choice. I think a more useful project might be to rate various items by champion. A possible way alternate way to do that would be to rank champions on how well they synergize with a given item.

This is derp. 'Nuff said. I like most of your stuff Diff, but this is just pointless.

well unless an item is rediculously cost innefficient then it is worth buying. You don't create builds from an item tier list. You build against what the enemy team that you are facing has. It's helpful to see a tier list but you have to remember that every item has it's proper use at the appropriate time. This is particularly true for tanky champs. AD and AP tend to build pretty much the same.

Nice attempt, but, in my opinion, this works only for standard represantatives of a certain role, which more or less build the same items in the same order. Item's strength and weakness depends on its effectiveness on the champion. For example manaless champions would not buy and RoA and DFG (they might start building DFG if the upcoming patch notes are accurate, though), while they almost always get a Rylai's Crystal Scepter. So this already influences the tier list's accuracy. 

Next, I kind of disagree with some item placement, and I certainly disagree with Wit's End being in Tier 3 of Magical Offensive, because it simply is not. Some other items are Ionic Spark because, even though it gives some magic damage, it is an on-hit effect for physical attacks, not to mention the attack speed it gives, thorefore it increases the physical performance not skill.

The hybrid items are pretty hard to deal with. Even though they give the specific stats, I think, these items should not belong in your division of items. No pure AP champion would build a Guinsoo's Rageblade because it provides too many unnecessary stats for the cost for the role. It's a speficid hybrid item that works on certain hybrids, but not all of them. Of course, many hybrids can also build non-hybrid items which benefit them well, so we get back to the point that item tier lists are very dependant on specific champions and the builds that work on them. 

I think the only way to measure item's quality is to take it apart and add a value to the stats to see how cost effective they are. From there you can sort of work around tiers for item's cost/overall stats/specific stat, or something.

I like this list a lot, but where is spirit visage? I know it's not the best item, but surely it can go on a few champions, right? Mundo, Vlad and the like.

Happy to help ;D

Added.

I quess probably (surely) few ppl told it already but: if you wan´t to make item tier list, it would take more work that this as items woks not only tier-wise but also role-wise,

-IE may give you highes raw damage of all AD items but only if it´s backed by another AS and/or Crit items, that´s OK if you are carry, not that OK if you are top-laner or jungler (crushing majority of them are AD´s too and how often you´l see IE on them? not very often), this alone pretty much this tier inaccurate ( if you have 3 ADs on your team and IE works only for one of them, how good it really is? :P)

-  throwing hybrid items into AP section is pretty unfair, they are meh on APs but great on hybrids, but Riot doesn´t gave us really that much of viable hybrids so either make them theyr of cathegory or take them our, it´s listing Shurelia as T3 tank item.

-some items provides both offensive and deffensive stats so giving them into purely def/off section bis bit misleading (Wit´s end is  Magic def T3 on APs T2 on carries T1 on bruisers etc)

so imho it would be better to sort items not by stats but by "clases" (carry T1: IE, PD, BT; T2 LW QSS...), you are already using notes on concrete items which is good and can be used for items that are more situational and/or good on certain champ(s) ( LW becomes T1 item if enemy team starts stacking armour; Trinity becomes T1 is champ sosses low-CD splells; WArmog becomes T2 if enemy posses %-based dmg- no it´s really not that good MR item agains Mal/Kog :P ). You´re list is most ikely correct gold-wise but certain champs has only certain choses in certain department (certainly bruisers has fewer AS items that he can acually use than carry, and yes i am certain about that) :P

Quoted: Defensive Physical - This is for negating magical damage or helping your team negate physical damage. It is not rated purely on the durability it gives you and other effects of the item will be considered. Quote.

Did you mean physical damage?

Regarding the item list, I think it's an interesting thing to debate about, although it doesn't speak much for itself since items are always extremely situational. I'm really grateful that you took the time to analyse the game and actually put this together though - most definitely an interesting read.

Fixed and thank you :)