I remember when this was a big question for me as well. The idea here, is to look at how the carry in question does their damage. Let me break it down a bit.
Firstly, you mentioned Ravenous Hydra. This is a no go. There are NO Melee ADCs, as part of the requirement to be an ADC is that they are ranged. I have tried to ADC with champions such as Gangplank, with poor results. So while Ravenous Hydra is an amazing item on Melee AD champions such as Talon, Riven, and Gangplank, we can't use it with ADC's.
Let's look at champions who deal major damage naturally, and have high AD Ratios. Some examples of champions like this are Caitlyn and Lucian. Either is viable in this case, but the large damage increase from a Bloodthirster is usually much more effective. Champions that scale well with AD benefit more from both the +70 AD Base stat, as well as the stacking passive that allows for up to an additional +30 AD. This leaves you with a potential +100 AD from a single item. This grants your champion both 100 damage on basic attacks, but also those 100 points of AD with which to make their abilities stronger.
Lucian's Piercing Light
Q, for example, scales with at minimum 60% of his bonus AD at rank one, and 120% at rank five. That's between 60 and 120 extra damage on the ability from one single item. As well, his Ardent Blaze
W scales with 60% of his bonus AD, granting that ability an additional In order to qualify as scaling well with AD, a champion should have at least a 50% AD or 60% Bonus AD Ratio on 2 abilities.
Basically, if the champion gets a high impact out of AD statistic, beyond the basic attacks, you want Bloodthirster for sure. These champions tend to be the ones that you don't just basic attack the enemies to death with. They make up for the lower attack speed, by having high impact abilities as well. Adding a supplemental Blade of the Ruined King, though, is not a bad idea if you have the inventory space for one, or if the situation calls for one.
Blade of the Ruined King
Champions that are about putting as many rounds down range as fast as possible, and champions that don't scale well, or at all, with AD will prefer the Blade of Ruined King. Some big examples are Vayne, Kog'maw, and Twitch. These champs like to get as close to that 2.5 attack speed cap as possible, so every item should help achieve that goal. Blade of the Ruined King, focuses on that with a buff of +40% attack speed, and the +25 AD, plus the passive, makes sure that your basic attacks still in fact hurt.
Champions that are focused simply on putting rounds downrange, usually apply some sort of de-buff on hit. Examples such as Vayne's Silver Bolts
W and Twitch's Deadly Venom passive. These champions care less about the damage that each shot does, and more about getting that de-buff on the enemy and triggering it. Another case is a champion such as Kog'Maw, who's Bio-Arcane Barrage
W deals a percentage of the enemy's health per basic attack. This synergies very well with Blade of the Ruined King's passive, which deals a percentage of the enemy's health in physical damage, in addition to the extra attack speed.
Lastly, there's the champions who just don't have good AD ratios. These champs generally focus on a sort of hybrid build between attack speed and damage, which again, the Blade of the Ruined King's +40% Attack Speed and +25 Attack Damage lends itself nicely to. These champions include champions like Tristana (Who has all AP ratios) and Quinn.
Then there's the situational buys. When the enemy team has a lot of beefy champions with a lot of Health, it's time to consider adding a Blade of the Ruined King to your build, even if you aren't using a champion that falls into one of these categories. That passive which takes 5% of the target's current health and turns it into bonus physical damage really helps against those beefy opponents. And for those times when you need that extra edge to get away from the guy you can't kill, it's active allows you to steal 30% of the target's move speed, and 15% of their maximum health.
Pro tip: Don't use a BotRK to deal with an opposing Dr. Mundo, as he'll just heal the damage right back. Instead, consider an Executioner's Calling.