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.