FTL is random, which means that everything, from the enemies the player will face in one star system to the exact configuration of the overall road he has to travel to deliver the message he carries, is different for each run through the game.
It also means that loot is assigned differently for every game and, while the randomness tends to be a reason for fear, getting a very good weapon or a subsystem early on can be a boon to one campaign.
The best loot I ever got in FTL so far (and I have played more than 30 hours of the game so far) is the Weapons Pre-Igniter and, in some important ways, my run through the game using it might have broken any future attempts.
In FTL jumping into a new star system resets weapons, which means that you have to wait for them to power up before engaging the enemy.
The Weapons Pre-Igniter changes all that and allows a ship to jump into a system with all weaponry fired up and ready to go, basically giving a player a pre-emptive strike which he can use to significantly tilt the balance of the fight.
In the early game, I was able to destroy scouts and fighters without losing any hull by simply disabling their weapons before they had a chance to use them and then by focusing on shields and on engines until they are destroyed.
As I got close to the final sector, I was able to use my pre-emptive strike ability to deal with cloaked enemies and even dangerous Slug ships easily, coming out battered and alive where otherwise I would have ended up destroyed.
The Weapons Pre-igniter would not have been enough to complete the final fight of FTL but, in conjunction with another bit of lucky loot, it allowed me to finish the game for the first time.