Naturally, I believe it is snorlax, at a whopping 1014.1 lbs. By naturally, I mean without abilities and items. With abilities, aggron is the strongest and heaviest with STAB for the move as well, making it very powerful
With 793.7 lbs or 360.0 kg Aggron is the heaviest learner of Heavy Slam. And with Heavy Metal, it can reach the double of that, becoming one of the heaviest of all Pokemon.