Excluding Megas:
The non-legendary Pokémon with the highest base Attack stat is Kartana at 181.
The legendary Pokémon with the highest base Attack stat (excluding Arceus, but that doesn’t change anything) is Groudon (Primal) at 180.
Including Megas:
The non-legendary Pokémon with the highest base Attack stat is Heracross (Mega) at 185
The legendary Pokémon with the highest base Attack stat is Mewtwo (Mega X) at 190
Source: Pokédex With Stats—Pokémon Database
Hope I helped!