Hello ladies and gentlemen. I'd like to make a post about a threat that seems to fall short of its full potential on this list: Tornadus
Let me first talk a bit about its strengths. As you might know, tornadus shines most of all because of two things. First of all there's its great speed stat which allows it to outspeed everything in OU that's slower than raikou, including prominent threats like the musketeers, lati@s, infernape and even gengar. What also helps with speed is its amazing ability: prankster. Add to this a movepool with okay supporting moves and you get a great supporter with prankster non-attacking moves like rain dance and taunt which allows it to, unlike other flying types, not be set-up fodder.
Another great move which might be underestimated on tornadus is tailwind. The only other viable prankster tailwinder currently in the tier is talonflame. However, even though talonflame has access to better STAB moves, tornadus has way higher attacking stats and also slightly better bulk. Next to that, tornadus has an actual fighting chance (hah, pun) against the rock-types in the tier with a powerful combo of air slash and superpower. Even without any attack investment, life orb superpower can OHKO ttar-mega after rocks.
Anyway, I think that, with a large enough niche in the current OU tier, tornadus fits perfectly in the B ranks. This is because, even though it works well on very offensive teams, its bulk is well below average in the OU tier, making it fall to strong non-super effective attacks in two times at most.
Let me just go through the description of the B rank and C rank to show that it is more deserving of the B than the C rank:
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche.
--> Tornadus can very properly fill an offensive niche in that it is the most viable tailwind setter in the current OU metagame who has a fighting chance against many of the tier's fast pokemon.
Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers.
A stealth rock weakness and not-so-great bulk definitely cripple it, but it's definitely not set-up bait for dangerous sweepers as it can taunt all of them, even other prankster taunters like sableye, and can tailwind the turn it gets knocked out to turn the tables.
Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
Offensively there are obviously better choices. However, it stands out as a support pokemon who can definitely pack a punch and can check many of the fast threats in the current metagame very well.
C Rank: Reserved for Pokemon who can be effective given the right support, but either have crippling flaws that prevent them from consistently executing their strategy or are typically inferior to Pokemon in the above ranks.
As said before, tornadus is a pokemon unlike any other and is not inferior to any particular pokemon in an above rank. With its prankster ability there are very few pokemon who can prevent tornadus from getting up a tailwind, rain dance or wreck up some shit.
Thundurus might seem like the better choice as it has a better typing and access to thunder wave, but it misses tailwind and has hammer arm instead of superpower, missing him the KO on many important threats and giving him a less-than-helpful speed drop. Tailwind is just as good as thunder wave in many cases, with the next pokemon not just being able to revenge kill the threat that would otherwise be twaved, but also giving it double speed for a couple of turns, making outspeeding even + scarfers a possibility.
Some calcs:
0 Atk Life Orb Tornadus Superpower vs. 252 HP / 0 Def Mega Tyranitar: 354-421 (87.6 - 104.2%) -- guaranteed OHKO after Stealth Rock
0 Atk Life Orb Thundurus Hammer Arm vs. 252 HP / 0 Def Mega Tyranitar: 296-354 (73.2 - 87.6%) -- 6.3% chance to OHKO after Stealth Rock
0 Atk Life Orb Tornadus Superpower vs. 0 HP / 0 Def Terrakion: 278-328 (86 - 101.5%) -- 50% chance to OHKO after Stealth Rock
252 SpA Tornadus Hurricane vs. 0 HP / 4 SpD Keldeo: 380-450 (117.6 - 139.3%) -- guaranteed OHKO