If you're looking for the Heaviest Non-Legendary Pokémon, it is Mega Metagross, weighing in at a total of 2078.7 lbs. However if you're looking for the heaviest Pokémon Overall, it would be Primal Groudon, with 2204.4 lbs. So if you're looking to trip the worlds heaviest Pokémon with Grass Knot, Primal Groudons your next Victim.
Please note, the publication Pokemon Deluxe Essential Handbook falsely lists Reshiram as having a weight of 6,727.5 lbs -- which would make it the heaviest Pokemon -- when in fact, its weight is 727.5 lbs as the video game itself states. Please note that as outlined in the link above, this handbook is known to contain several errors and is written and published by a third party, not Nintendo or the developers of Pokemon. Additionally, note that Cosmoem and Celesteela have tied with Primal Groudon as the heaviest Pokemon since this answer was posted.
Hope I Helped!