I despise Heatran. Partly because it looks hideous, but mostly because in every battle I did on ShoddyBattle before I quit that game half of my Fire attacks would be absorbed by a switch-in Heatran.
I do not mind it if one Pokemon consistently defeats me, but if that Pokemon is a legendary, then it gets extremely annoying. It felt like every opponent had their own personal Stark Mountain in their backyard.
Though, to be honest, I do not hate Pokemon as much as I hate the way they are exploited, but that does not change the fact that Heatran (and Garchomp, too) are the only Pokemon that make me cringe.