Fire Fang,Lava Plume,Fire Blast are some good attacks from Entei.

Canon main game wise:
Palkia's Spacial Rend cuts through space
Dialga's Roar of Time is said to stop time
Arceus's Judgement should be more powerful since he made Dialga, Palkia as well as Giratina

Game Mechanics has Explosion being the strongest with base 250 attack power but they ignore canon.

Side Game wise: Rayquaza hyper beamed a life wiping meteor(best destructive feat in pokemon)

Anime/Movie:Tackle with ease... wait what!!! Now let me explain my reasoning for this. We all know in Pokemon Movie 12: Arceus and the Jewel of Life there was the meteor that would have wiped animal existence off Earth for at least a few million years. Now Arceus was said to have attacked that Meteor head on with its plates. This would connote that it was only a select few moves like Tackle, Take Down, Double-Edge or maybe even quick attack. Since it never said he not damage from using his attack it would be safe to say its not take down or double edge. Secondly nothing was said about him going any faster then normal IIRC so that would only leave tackle as the attack he used on that meteor. Now here's were it becomes inconsistent, with only a few plates missing, he uses his signature move Judgement. Now you would've thought it would be amazing since his tackle destroyed a life-wiping meteor with only some recoil from the force of the meteor, agreed? What did we get? a spammed blast attack barely damaging the city block sized area he was in on his return.

long boring explanation done.

I actually agree with the ones LostOblivion said.

Roar of time dialga,spaciel reind palkia,jugement day arceus and giartinas move i forgot it sorry) and mew and mewtwo moveis

i think hyper beam is the strongest move, but I'm not sure. oh and here's a pic of Empoleon using hyper beam.

Arceus's judgement

wow that's such a nice picture Harpaw8!!! yeah, judgement, looks way stronger than hyper beam:)