Heatran Movesets

Choice Scarf-Naive/Rash Nature-Choice Scarf
Fire Blast
Earth Power
Explosion
Hidden Power Ice/Grass/Dragon Pulse

Heatran is a great Pokemon to take advantage of Choice Scarf Because its only below average stat is Speed. This makes Heatran an excellent Revenge Killer. Also with the ability Flash Fire, Heatran can freely switch in on any fire move and have his own fire move doubled in power. For the move slots, Fire Blast Is a much more powerful option but has a 15% chance of missing so feel free to opt to use Flamethrower for its accuracy. For the last Slot Dragon Pulse and HP Ice can be used to eliminate Dragons while HP Grass is super effective against Water and Ground Types like Suicune and Hippowdon.

Choice Specs.-Modest/Timid Nature-Choice Specs.
Fire Blast/Overheat
Earth Power
Hidden Power Grass
Dragon Pulse/Flamethrower

This set gives Heatran amazing attacking power. Fire Blast destroys anything that doesn't resist it. Unfortunately it's 77 base Speed really hurts it so Timid Natures probably your best option. Fire Blast is a great attack and has the advantage over Overheat that it doesn't lower your Special Attack but look out for the 75% accuracy. Earth Power destroys Ground and Rock types that switch in on you. HP Grass also covers those types and Water. Dragon Pulse KO the popular Dragons but Flamethrower is a good option for a more reliable fire attack.