Who can do the most damage in one spell?

That would technically depend on the mechanical maximum for damage numbers. It would be either Nasus, Veigar, or Garen (against a Sion) simply due to the scaling, and it would take real life days of stalling a custom match or digging through the coding that is League of Legends to figure out which caps out last.

fully charged nunu ult with

i think it could be garen because his ult scales off missing health and isn't capped like riven's or darius's. Yi's Q can nearly oneshot someone also, but i think theoretically, if you ran scaling ad runes on riven, bought4 ie's, a triforce, and a last whisperer, her auto's could one shot somone instantly due to the spell blade passive and her runic passive. I seen full crit riven one shot some bots a while back also, so damn scary....

Lucian's ultimate if you build to maximize it and you hit someone with every single shot of it.

Nunu R

Annie's Tibbers scales off 980% of your AP paired with the aura O.o

My fist.

Heimerdinger's Q, since it does infinite damage if not destroyed.

R: 400 / 600 / 1000 (+ 160% AP) + W passive R: 625 / 875 / 1125 (+250% AP)

Per target. Your math is wrong for MF.

I think its Syndra R with 7 Spheres, Muramana active, and a full mana build gonna edit in the math

Base damage (7 spheres): 1260 (+ 140% AP)

Build: Muramana, Seraphs Embrace, Frozen Heart x2 (40% CDR is required to get 7 spheres), Rabbadons, RoA Mana : 1184 base, + 1000 + 1000 + 400 + 400 + 650 = 4634 mana AP: (80 + 60 + 120 + 139.02) * 1.3 = 518.726

damage from AP: 518.726* 1.4 = 726

damage from muramana = 1578

total damage = 3564, plus its mixed it might not be the absolute most, but its pretty damn far up there

mf does not deal damage per ball, but per wave, so 325x8 +w x 36(asuming 0 stacks at first hit) i think rammus has the highest, followed by fiddle(rammus got 1560+2.4ap, fiddle 1625+2.25ap, mf 1000+2.16ad+1.6ap, nunu 1125+2.5ap)

vs a single target, its lucian ult

Correct me if im wrong but isnt it annie with summoning tibbers. His aura has a total ap ratio of like 980% for his full duration.

It is Lucian, for single target.

Maximum Total Damage: 1170 / 1650 / 2145 (+650 / 750 / 825% bonus AD) (+260 / 300 / 330% AP)

Counting AoE, it is Fiddlesticks on Hexakill.

Total Magic Damage: 625 / 1125 / 1625 (+ 225% AP)

Veigar. I've had over 1k ap on the yordle and could oneshot the adc with dfg and 'q' and autoattack (lichbane) alone.

Q is his lowest ratio.

Also, he's an infinite scaling champion. Like nasus and thresh, he will scale as long as he keeps playing right. Though he will likely have the most power in one hit among them.

I wonder how much Sion can deal.

With one spell that isn't channeled?

AP - I'd say Veigar.

AD - Definitely Lee Sin or Zed, but probably Lee Sin since Zed's requires further commitment.

Technically, annie's ult, probably. She casts R, Tibbers attacks you for however long he's alive, that's one spell. Imagine how much damage Tibbers can do to one person over the duration of his life. That massively outscales any of the other ults (Nunu, wukong, kat, MF, Veigar)