Time to start posting around here I guess!
Here's a lead set I have been running for a while.
Rayquaza @ Focus Sash
Ability: Air Lock
EVs:
252 Atk / 252 SAtk / 4 Spe
??? my bad nature (
+SpA / - SpD)
- Draco Meteor
- Earthquake
- Flamethrower
- Extremespeed
Against the top 10 leads:
Deoxys-e - Draco Meteor and ExtremeSpeed will KO.
Groudon - Rayquaza outspeeds and 2HKOes 252 HP / 0 SDef Groudon with Draco Meteor.
Darkrai - The best thing to do here is to ExtremeSpeed and switch, or just switch. Darkrai is mostly annoying due to Dark Void.
Deoxys-f - ExtremeSpeed twice.
Kyogre - Draco Meteor followed by ExtremeSpeed does a good amount of damage to 252 HP / 0 (S)Def Kyogre, 75% on average. If it isn't running HP, it takes 88% on average (which means it can't switch if you get SR down).
Dialga -Earthquake + Earthquake will KO. (Too lazy to run calcs but EQ + ExtremeSpeed might even be able to kill if using a +Atk nature, which would be very good.)
Mew - Lead Mew are less worrying because sometimes they Taunt (expecting Dragon Dance or Swords Dance possibly). Draco Meteor does 61%-71%, and 30%-35% on the second. It's possible but really not guaranteed. It's likely Mew will either U-turn or Explode on the second, though, so sometimes using Earthquake is better. Rayquaza comes out on top thanks to Focus Sash, basically.
Shaymin-S - Flamethrower and ExtremeSpeed will KO Shaymin-S. The only threat to this is if Shaymin-S gets two flinches, finishing Rayquaza on the third (31%~ of Shaymin-S coming out on top).
Mewtwo - Draco Meteor and ExtremeSpeed will KO 4 HP / 0 (S)Def versions.
Forretress - Flamethrower.
As you can see, only 2 of the top 10 leads can consistently beat Rayquaza. I've mostly been running this set because my team has tons of checks to Darkrai and Kyogre, so generally it's fine for myself at least.
The main reason I used Rayquaza, though, is because it can come in mid-game very often. Deoxys-e, Groudon, Deoxys-f, Dialga, and Mew are the only ones able to set up Stealth Rock on Rayquaza. Interestingly, 4 of these 5 are 2HKOed by Rayquaza. Because of this they get an ultimatium: bring Rayquaza to 1% and get no Stealth Rock, or use Stealth Rock and leave Rayquaza with plenty of health. Either way, it often allows Rayquaza to come in mid-game and spam a quick turn of Draco Meteor, or use ExtremeSpeed against any annoying low HP Pokemon (Darkrai, Latias, Garchomp, etc). Against the ones who don't set up Stealth Rock, the player just needs to predict when to switch in Rayquaza safely (ie. don't let the opponent set up Stealth Rock without getting Rayquaza into play).
Just thought I'd share this awesome lead - it's been serving me well.