Arceus is said to be the strongest, in the mythology and battling.
Arceus is also known as "The Original One", as it is said that it created Sinnoh and Ransei, and possibly the entire Pokémon universe, along with the lake guardians and creation trio.
And Arceus is the best battling because well, for starters, it can be any type. Plus, It has the highest Base Stat total at 720, and 120 in each stat, making it somewhat overpowered. Not to mention its massive movepool, rivaling that of even Mew's, even with some rare and powerful moves such as its signature move Judgement, and ExtremeSpeed. And the boosting moves it gets in Calm Mind and Swords Dance make it a huge threat to any Pokemon.
Arceus can only be found in the wild legitimately with the Azure Flute, which was never distributed at any official event and can only be obtained by cheating. However, Arceus itself was distributed via multiple Nintendo events in 2009 and 2010 and the Pokémon Global Link in 2012.
Check here for the events.
Hope I helped! :)