Oh boy, this is gonna be hella long.
I'm gonna talk about it very generally and talk about some masteries I think are just trash compared to the other, and this is for pre patch 6.1 but I will address post 6.1 patch later
ADs (not just ADC): Left (wanderer), right (assassin), left (merciless), right, left, thunderlords. (In order from top to bot)
I don't see any reason to be taking savagery especially in ARAM, you're not farming. And in 5v5, its just more worth to get to places after basing as fast as possible, getting there 0.3 late could make you miss a siege minion. You could take savagery if you're not confident in cs-ing but that is easy to practice.
I play carry so I want to maximize my damage as much as possible so I take assassin, I believe a rioter told us the range is 800 (or something) so its quite small so getting the 1.5%+ damage is pretty nice. In aram I still take that because 20hp on biscuit? Really? 50g for a hp pot is just not worth at all, I barely buy potions after 1st or 2nd back. You deal more 1.5% damages than you can recovers 20hp on a 50g hp pot.
Merciless = more damage. I don't think there will ever be a day where you choose sustain over more damage, because if you're AP you can get tear or athenes or whatever, if you're AD you shouldnt be spamming spells or else you'd have essence rever.
Dangerous game, obvious one here.
Precision, obvious here too.
Thunderlords, it was the best mastery for damage of non-pokers, but now all you have to do is just take points out of keystone and precious and put it into battering blows and the keystone you want.
Which keystone? I'm not sure yet, 6.1 just came out, but ideally, if you're a crit ADC you take warlords, if you're a bruiser or tank that went down the ferocity tree = fever of battle. No death fire touch for ADs, except of AD pokers, maybe Ezreal, but other than that nope. Stormradiers surge could make a surge in people picking this for assassins but as I said before, why not just get more damage? Utility doesnt kill someone, damage does. High comboers take thunderlords like Lucian (quick and easy trade in lane).