This is my rating of all the systems from FTL
Weapons-A+- You NEED these to win the game and a lot of other fights, these are pretty self explanatory to be honest. Piloting-A+- It helps you move around and powers engines, which can help you evade enemy shots and it doesn't require reactor. Medbay-A+- It helps you keep your crew alive, and most of the time its not powered, what makes it better than cloning is that you can power it on and send your crew there to fight boarders and they pretty much can't die. Cloaking-A+- You NEED this, it is cheap to power but not to buy, it adds 60 to your evasion and is so helpful Hacking-A- Its very good for the flagship and all fights alike, its only main issue is that defense drones can shoot it down, and there augment preventing it. Shields-A- I would put shields in A, but they require SO MUCH POWER to operate, but at the same time you need them for lasers. Engines-A- They let you evade enemy fire, its only downside is that they need two people to operate them Drones-A- Drones are better than mind control even though they aren't as reliable, they have wider use than mind control and more varity Backup Batter- A- Dirt cheap and has a nice bonus, I typically don't use it, but it has been a great help in the past Teleporter- B/A+- This has 2 ratings, B is a 2 tile teleporter A+ is a 4 tile teleporter, I gave it B even though you get more scrap and higher chance to get crew/weapons from the ship, Your crew has a high chance to be killed, and they only level up fighting, but its totally worth sending your crew to die for the Federation with a 4 tile teleporter Mind Control-B- It has its moments, and is more reliable than drones, but it just come short compared to drones, and it isn't that expensive either Doors-B- I very good against fire/boarders and doesn't require power to operate, and is fairly cheap to upgrade, but if you have the right crew setup you don't need to worry about any of those Clone bay-B- Its nice that your crew can't die, but I find medbay a lot better at dealing with boarders, and its better to have your crew ready to go before a fight rather than half dead Artillery-B- You can't buy and is only on the federation ships, its comes in handy for must fights but seems to slow, Sensors-C- These are the most useless system in the game, I would not upgrade these or even buy them unless you have spare scrap O2-Situational- If you have all lanius then there no point in having it on, but if you have crew that rely on it you need it