Its impossible to have the strongest deck since deck types cancel each other out. For example power decks are weak against defensive decks, defensive decks are weak against burn decks and burn decks are weak against healing decks and healing decks are weak against power decks. But if you really want strong deck then here is an infernity deck:
Effect Monsters
Synchro Monsters
Traps
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yes there is
Well, the minimum in a yugioh deck is 40 cards maximum 80cards. Most people try to have a smaller deck over a larger deck, to get cards they need. About a 40-50 card deck is good.
Make the deck you want as a recipe. Then go into the deck menu at the top left corner of the screen. Then you press "Save recipe". Then just press one of the spaces. You can do this over ones you've already got but this will overwrite them.
You pray that you win by a lucky break, because his deck is so broken for what you have at that point that no strategy exists to help you win.