Alrounded that is. Not just Atk
Statwise, yes. Slakings monstrous 670 base stat is higher than many legendaries and its movepool is both diverse and intimidating. However, Slaking suffer from its ability, Truant. Since it can only attack every second turn, it's stats is effectively halved to 335. It is also easily countered, being completely walled by Protect, and being setup fodder for Sword Dance and Subsitute for example. Slaking Is not the strongest non legendary Pokemon ever.
Rampardos currently has the highest Attack stat of all non-legendary Pokémon with 165.
Source
Slaking's attack stat is 160.
Serebii list by attack stat
However, Slaking has the highest base total of any non-legendary Pokemon.
Serebii list by base stat