Still don't get what strongest pokemon is

if you're asking for a playthrough rather than in theory: pick one of the pseudo-legendaries (garchomp, dragonite, tyranitar, metagross and so on) and you'll steamroll most of the story. 600 base stat total, no legendary needed.

depends what you mean by strongest. lore says arceus, it made the universe. on raw base stats it's mega mewtwo and mega rayquaza at 780 - eternamax eternatus is higher but you can never use it. in actual battles there isn't one answer, it changes every game with the meta. which game are you playing?