What is the damaging skill with the absolute largest AOE and possible runner ups?
You can also include combos with devotion skills, but i want to know the largest possible AOE size in meters for a single skill.
It’s probably Flash Freeze. War Cry also has a massive AOE
Tainted Eruption and Nightfall are fairly large as well from what i remember.
Anything besides those two? War cry only has the health reduction, is that good for wave clear? like trash mobs or spreading aoe devotions?
Large doesn’t mean best in this game. You won’t be able to pick a large AoE spell and just hope to spam it and win.
As for the largest, War Cry is the only one I can think of.
Something important War cry has is %reduced target Damage for 5 seconds. Break Morale has flat target Physical Resistance for 5 seconds.
The cooldown on War Cry means it’s good for proccing devotions with a low chance like Eldritch Fire.
Besides the ones above (also Nightfall has a smaller AoE than i remember so disregard it), I forgot to mention Canister Bomb which is capable of spreading mayhem and from what i’ve heard is capable of proccing devotions multiple times with it’s fragments.
Curse of Frailty also can proc devotions over it’s duration more than once and is more likely to do so on large groups and has a pretty big radius.
I just want to know the fastest way to cover the screen with a single or a few clicks i forgot about curse of frailty proccing devotions multiple times, and canister bomb as well. I’m not even entirely sure what im looking for, but i just want something fun and huge doesn’t necessarily have to do insane damage.
And thanks for all the input so far. 
Try devastation/anasteria devastation/meteor shower combo coupled with few more proscs and CD based skills its amazing in terms of both dmg and AOE.
Does cooldown increase proc chance or something?
Yes. I’m not sure on the exact equation but cooldown and and i believe duration scale the % chance on celestial powers. I think i’ve seen Ceno post the equation on here somewhere but can’t remember what or where it was.
I’m pretty sure cooldown also lowers the internal cooldown for devotion abilities, like if you have one equipped that has a cooldown of 5 and you equip a tome with 18 cdr it should lower a bit.
So far from Grim calc here are the sizes of the two largest AOE’s we’ve talked about.
AOE
Flash Freeze 12/12 16 Meters 22/12 18 Meters
War Cry 12/12 16 Meters 22/12 18 Meters
3rd Runner up?
War Cry and OFF share their radius scaling.
Canister Bomb has a variable but potentially very large radius as well, and while it doesn’t do damage, so does CoF.
Bloody Pox theoretically has an infinite radius.
If you’re just looking at a skill or proc’s meter radius, some others that i’ve found are 20/10 Curse of Frailty at 13, sort of Apocalypse if you count the target meter radius at 12 and “The Big One”, 10/10 Curse of Frailty and Tainted Eruption at 10.
It’s been a while since I last played my Quick Jacks Sorcerer, but if you have the energy, you can cover a large part of the screen with QJ.
Best way for this is devastations, as JoV said, and CoF/Bloody Pox+Eldritch Fire.