Evaluating the entire USM metagame, it's Heatran. Evaluating the entire SM metagame, it's Toxapex. It's really hard to ignore either Heatran's sheer utility and wallbreaking throughout USM or the absolute stranglehold position Toxapex took in the SM metagame. So ultimately, it comes down to a tie between Heatran and Toxapex, though for the sake of a choice and decided by a coinflip, I am going for Toxapex.