This is sort of a simple idea, just name the situation that you think would produce the most powerful attack imaginable. Here's mine:

Overheat, having previously used Sunny Day, done by a fire type Pokemon holding charcoal and with a nature that increases special attack, performed on a Scizor. I haven't done the exact math, but that roughly hits about 1000+ damage.

Discuss.

Hyper Beam used by an Adamant Rayquaza holding Choice Band with 252Atk EV's and 31Atk DV's having previously used Dragon Dance 6 times on a Chansey when the Chansey has a Hasty nature with 0 EV's and 0 DV's.
Let me check this on the damage Calculator.
EDIT:The DC said that it does an average of 5895.23 WITHOUT THE 6 DD's.

blastoise using hydro cannon on a rhyhorn while rain dance is in effect and nature that boosts special attack and maxed out sp.attack EVs and used calm mind as many times before its PP ran out.

EDIT: ok, I changed it to blastoise. Happy?

Er, Suicune can't learn that.

Well. I suppose it would have to be this:

Opponent is a level 1 Happini (newly hatched) that has had its defence lowered by six.

Regigigas, who has been Baton Passed x3Swords Dance holding a Choice Band uses Explosion.

Oh yeah, the Regigias is Adamant with 252 Attack EVs, and has 31 in Attack. And the Hapini is Hasty nature with 0 DVs in Defence.

Ooh, that feels good (in my mind)...

Beat that.

Hahaha, wow, you guys are even more vicious than I had suspected.

I did have equal level and stats in mind, but you guys made it interesting, so go with it.

I wasn't even thinking about moves like dragon dance, etc., but great ideas nonetheless. You've made my Overkill Overheat look puny by comparison

A lvl 100 Mewtwo with a nature that boosts speciel attack and is holding a choice band that was trained with a macho brace with the max EVs and DVs on its speciel attack while previously using calm mind a-bizillion times and then uses physic on a lvl 1 croagunk with 0 DVs and EVs on speciel defense with a nature that lowers speciel defense and it had it had its speciel defense lowered 1000000 times.
Beat that, UBER POWER!

Um...you can't lower a pokemon's stats that much.
*Uses scarcasm dedecter that I borewoed from ~gardevoir* Whoa! This baby is of the charts!

I have one. A Ninjask using Swords Dance 6 times then Baton passing to a slaking with adament nature and attack EVs maxed out and then the slacking uses a Hyperbeam/Giga Impact on a Chansey at level 5 with a nature that decreases defense and no EVs.

the slaking could use flail at low hp as well. actually reversal would work even better since it is supereffective against chansey/happini.

i think the regigigas one is the top. STAB explosion is hard to top. its a pity explosion never gets stab.

Maximum times you can increase or decreasde a STAT is six. And weather it is EV trained with or without a Macho Brace it has the same effect.

mew2 used calm mind 6 times on aa lv 1 crogunk with no evs or dvs and a nature that decreses sp defense and mew2 is holding a twisted spoon and it uses psycho boost and gets a critical hit WOW

Mewtwo cant use Psycho Boost.

It doesn;t? Hm, I always thought it did...

And guys, please can we keep it actually possible. No Mewtwo using Psycho Boost or Calm Minding x Infinity sort of stuff. I would imagine Prophet Hobo would prefer it realistic.

Here's another one:

Marowak, with Adamant Nature, 31 DVs into attack, 252 attk EVs, holding Thick Club, using Giga Attack that has been boosted by 6 Swords Dances. Oh, against a lv. 1 Happini with 0 Defence EVs and DVs, and a Hasty nature that has had Screech used against it thrice.

Don't who would win out of Regigigas and Marowak in that respect.

Regigigas has slow start so it wont do much as compared to the Marowak

Eh, Marowak wouldn't do much of anything (well, of course it would still KO the rather pathetic Happiny you have there). If I recall, because of the Thick Club, after (assuming you have all the EVs and DVs in Attack, and the nature) a few Sword Dances the attack loops over the max resulting in a low attack stat. I think I got that off of a NetBattle related thingy, but I can't remember where exactly. Heheh.

-Gardevoir~

I've always thought of ultimate damage like this:

Lv. 100 Pikachu, full stats in special attack and whatever, holding a light ball. He somehow has special attack at stage 6, using baton pass or some other bulking method. Pikachu uses charge acquired through breeding with a plusle. He used thunder on a pelipper. Pelipper is using fly, and rain dance is in effect. Pikachu scores a critical hit. Using Pelipper's base Special Defense, that's an average of 40906.83 damage. Nice.

By the way, maximum damage would be 44200.2

blaze

Stabed explosion = 375 Stabed blast burn + sunny + Blaze = 506.25

And also remember that explosion wont go super effective on anything.

Damage calculator average thingy of my lvl 100 Charizard (max special attack) against a lvl 100 paras (min sp def): 308 188276.46 .....the circunstances are posted above.

What about Defense Curl + Rollout?

That's a possibility.

An Modest Nature Lv.100 using Thunder after using 7 Metalsounds to a lv.2

7 metal sounds wouldn't all take effect. I'm not sure how many stages metal sound lowers special defense, but I think it's 2. If so, only 3 would work.

Even if it's only 1, you can only lower the stage 6 times.

Overheat used by... a Modest Blaziken @ Lv100
-STAB
-with max SA
-attached Charcoal
-Sunny Day in effect
-Blaze is activated
-CHit
-On a Lv100 Paras, with the possibly lowest SD

ON DAMAGE CALCULATOR, IT SAYS IT WILL DO AROUND 5870 DAMAGE! WOOT?

Bump that up by making the Pokemon a Ho-oh with 3 Tail Glow's baton Passed to it after using 3 Metal Sounds against the poor Paras.

BURN!