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Hey, thanks for clicking on my guide, just a brief intro to this guide. Be sure to read the Notes for the build.
Please note that the builds, runes, and masteries, may not be the ideal ones for the champion they're listed under. They're just generic examples of runes/items/masteries that can be used on the majority of ADCs.
AS A SUMMARY THIS IS NOT A GUIDE ON HOW TO BUILD SPECIFIC ADCs BUT A GENERAL GUIDE ON HOW TO BUILD ADCs

Any item that builds out of Zeal is pretty much core on any ADC except Ezreal, Miss Fortune, and some others. The mobility, attack speed, and crit chance it offers is too great to pass up. Many times, it'll be a good idea to just build the Zeal and start building another item. Under most circumstances, you will not be fed enough to buy an Infinity Edge immediately after building your Phantom Dancer or Statikk Shiv. Consider how much gold you have and what item you need. For example, if you have Bloodthirster and a Zeal, and the other team is starting to buy armor, it would be a good idea to sit on your Zeal and start building your Last Whisper, finishing the Phantom Dancer or Statikk Shiv later.
Whether you build it into a Phantom Dancer for the late game or a Statikk Shiv for the early game burst damage and poke is up to you.

Infinity Edge is the highest damage output item in the game, this is definitely a must buy on pretty much any ADC. The stats that the Infinity Edge gives are all increasing your damage, that's 4050 gold worth of pure damage. This makes it the highest damage item in the game. (Note: Bloodthirster with full stacks has 4720 gold of stats, but only 4000 gold of damage making the Infinity Edge a better choice for a second damage item, keep in mind that Infinity Edge's passive adds in even more power, that can't be measured in gold).

Everyone will build armor, and if they don't, then a Last Whisper will make your autoattacks pretty much deal true damage. If an enemy champion you're focusing as the ADC (most likely the tank) builds, for example, 200 armor, you'll deal 1/3rd of the damage you'd usually deal to them. However, after building a Last Whisper your autoattacks will deal a bit less than half (~45%) the damage you'd normally deal to them. With roughly 300 AD, that's about an extra 50 damage per autoattack, not counting crits.

After getting a full build and enough gold, you should sell your Berserker's Greaves to buy a Zephyr. You do lose some movement speed, but you gain an extremely high amount of stats in return.

Unless your lane opponent isn't harassing you at all or you have enough sustain to outsustain their harass, an early Vampiric Scepter is always a good buy.
Doran's Blade is has good power at a low cost and low sellback price. It's an early game investment that delays your core build a bit.
I'd see no reason not to start Doran's Blade unless you're not using lifesteal quints, in which case, you should buy some. A second Doran's Blade in the laning phase will give you a larger increase in power and sustain, without delaying your core build too much, compared to a laner who chooses to buy a Long Sword or Dagger and 2 Health Potions with the 475 gold instead.
I'd refrain from buying Berserker's Greaves in the laning phase, I believe the laning phase for bot lane usually comprises of short-term trading where each side pokes each other with an autoattack while cs'ing, not all-in fights. In addition, the extra AD and sustain provided from something such as a Doran's Blade or Vampiric Scepter allows you last hit better and stay in lane longer to get more last hits.

I think of it like this:

Bloodthirster is great with ADCs who have high AD ratios, some examples of this are Draven's Spinning Axe, Graves' Buckshot, and Vayne's Tumble. These champions with spammable moves that have high AD ratios makes Bloodthirster a great choice for them.

Blade of the Ruined King is more cost efficient than a fully-stacked Bloodthirster if the target being killed has 3000 health or more. It deals more damage than Bloodthirster with full stacks against targets with 1500 health or more. This means that if the enemy team is just stacking health like they did in the Warmog's Armor meta, then build a Blade of the Ruined King. I personally do this when they have 3+ people on the other team with >3000 HP, or if they have a mundo/cho'gath/other health-stacking champion.
Don't forget about the active either, if you feel like you need self peel more than Bloodthirster, by all means, get a Blade of the Ruined King.
Another thing about the Blade of the Ruined King, the bonus attack speed it has makes it the better option for champions with on-hit effects such as Vayne's Silver Bolts, Varus' Blighted Quiver, and Kog'Maw's Bio-Arcane Barrage.
The last benefit to the Blade of the Ruined King I can think of is the build path. Compared to a champion building Bloodthirster, champions who build a Blade of the Ruined King get a huge power spike as soon as they build the Bilgewater Cutlass, giving them an edge in lane against someone who just bought a B. F. Sword or Doran's Blade and Pickaxe.

Here, I list a bunch of defensive options and when to build them

Guardian Angel - The go to defensive option, if no other defensive item is better. Once the passive on this is used up, sell it to buy a different defensive item. A Guardian Angel won't save you from being bursted down, but in a teamfight, it allows your team some time to peel for you while you're reviving. A Guardian Angel is generally purchased against high burst assassins that can't do much after bursting you (For example a Katarina or AP Master Yi not getting their resets, or Zed)

Frozen Mallet - If the enemy team has no real burst threat, but you do need help peeling for yourself, build this. I find this item useful against people with small dashes like Renekton or people that are prone to kiting like Darius, but not people who can just jump on you like Kha'Zix or Zed, when playing against these type of champions, I'd rather get a Guardian Angel or Warmog's Armor since the chance of getting bursted down without the chance to peel for yourself is extremely high. When you're in a winning situation and can't die easily in teamfights because of your team's peel, Frozen Mallet can be purchased to apply a slow to targets, making chasing them and cleaning up after a teamfight easier.

Randuin's Omen - Build this for when the other team dives you, but can't kill you immediately due to having high DPS but low burst. This is extremely useful against champions like Tryndamere and Shaco since the passive of Randuin's Omen will slow their attacks, lowering their damage output and mobility. Although this is primarily built on tanks, its a good choice for ADCs in certain situations too.

Warmog's Armor - Warmog's is a great choice for sustained fights where you're taking constant damage like Defile or Rampage. Whenever I get enough gold, I'd sell my Guardian Angel for this item.

Banshee's Veil - If the other team has something to initiate on you such as Bandage Toss or Unstoppable Force, build this. I tend not to build this because of how easy it is to pop, and generally speaking, Mercurial Scimitar would be better. However, if there is a time when there's an initiation skill like Unstoppable Force, Curse of the Sad Mummy or Glacial Prison running around and there's no way for the other team to pop your Banshee's Veil passive, then this would be a great choice.

Mercurial Scimitar - This is for Nether Grasp, Infinite Duress, Terrify, and puncturing taunt. Basically if you're up against some kind of CC that you'll die to if you can't escape from it, you'll want a Quicksilver Sash if you don't have cleanse or the CC you're facing is suppression. You shouldn't build a Mercurial Scimitar immediately, it should be your last item, there isn't any reason why you can't sit on the Quicksilver Sash.

Maw of Malmortius - Against an AP champion that can burst you down such as Diana or Annie this item will prevent them from 100-0'ing you with their immediate burst and gives you some damage too. I find this to be a situational item that I'd build if you're just dominating but there's some fed AP burster running around. To me, this item is a Warmog's Armor with AD that only helps vs. AP.

I hope you enjoyed reading my guide on ADC itemization. Remember, these are only guidelines, there's many situations where items can be better than others that I didn't list on here. Use your best judgment to decide what to buy.