This is a rather big nomination, but Heatran for mid S rank . Let me explain:
S Rank: Reserved for Pokemon that are amazing in the metagame. These Pokemon are usually able to perform a variety of roles effectively, or can just do one extremely well. Their use has low risk involved and high reward exerted. Pokemon in this rank have very few flaws that are patched up by numerous positive traits.
When it all comes down to it, Heatran is definitely one of the best pokemon in the metagame. It is able to go both offensive and defensive extremely effectively.
Defensively: Heatran's typing is absolutely perfect for AAA. After Levitate, it has a whopping 9 resistances and 2 immunities, most of them prominent attacking types (Dragon, Ice, Fairy, Flying, Normal, Ground). With this typing, it can effortlessly wall many of the premier threats in the meta, such as Mamoswine, Cresselia, and Garchomp just to name a few. Or it can opt for a water absorb tran that could give it a form of recovery outside lefties and tank the water/ice coverage that we see so much. It could possibly even run a regenerator set if need be, which allows it to form an extremely annoying core with something like Amoonguss; or maybe a prankster variant that sets up rocks and burns things; the possibilities are (almost) endless. 90/106/106 bulk is pretty good, combined with access to moves such as will-o-wisp and roar to further boost its utility.
Offensively: Of course, if Heatran were only a defensive pokemon, it probably wouldn't deserve S rank. However, recently, ChloroTran has been getting more popular, as it deserves; ChloroTran is one of the scariest offensive threats in the metagame. Its sun boosted Eruption 2HKOs things like Chansey and Suicune, and OHKOs things like Azumarill (after rocks) and Keldeo. The amount that ChloroTran has affected the meta is probably enough for Mid A rank or even A+ rank. The fact is, the thing is almost impossible to wall (outside of things like AV regen Goodra + Regen Chansey core) without flash fire, and even then, a well played ChloroTran destroys teams. Not to mention that its typing allows it to tank Refrigerate Extremespeeds and Pixilate Extremespeeds with ease, kill gale wings users, and not care about toxics and will-o-wisps that otherwise put you on a timer. And with far more reliable sun setters in the tier as opposed to OU, keeping sun up for this thing isn't really a problem.
Of course, maybe Mid S rank is still a bit too much for it, we'll see.