I rather ignorantly consider Mew the strongest pokemon ever. The original rarest pokemon ever, i won't let Nintendo or Game Freak come along every generation and go 'Oh, no, heres the new strongest rarest pokemon ever' *cough* Arceus! I've heard/seen in some places/pokedexes Mewtwo being stronger than Mew, Mewtwo is a clone so i consider him weaker.
Celebi, Jirachi and Mananphy, i consider them all, 'The Mew of that generation'.