Many people believe that the most cost efficient item to max damage is the B.F. Sword. Although this is true to an extent, some champions benefit more in DPS from the similarly priced Zeal + Dagger. I tested a level 7 ashe (no AS steroid but high base AS per level, no onhit) to a level 7 kog'maw (passive AS boost, 5% hp dmg onhit @ 7) typical 21 offense masteries and flat AD runes
At this time if you've been farming okay, a 1600 gold isn't hard to get.
The results : Ashe benefits from the B.F. Sword as optimal first item to max damage, BUT kog'maw actually gets more DPS from the zeal + dagger.
The lesson is this: the optimal build order differs between ADCs.
I would say, with the current meta of starting long sword, first full item should definitely be blood thirster, just to get the most DPS and win lane. I've had this tested with Draven, Graves, and Quinn. It works for most other ADC's, but I would recommend something like Infinity Edge on Kog'Maw and/or Vayne, simply because they don't have good spell scalings and the crit when you do % health damage is pretty good. --- Syndra is my waifu. "My power is limitless!"
As others have said, it is completely situational.
I'm a fan of going BT/IE > Shiv/PD > BotRK/LW with each purchase being dictated by the previous one AND what your opponent is building. If they are building armor because they are afraid of BotRK get a LW instead and poop on them. --- ~~SUPER SONIC FIGHTER~~ Sonic the Hedgehog FTW!