I would like to make a change.
Azelf: C- ---> B
I would first like to base my change on the description of the rankings. I apologize in advance for the long post :)
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. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outperformed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
I won't pretend like Azlef has flaws; Let's review them. 75/70/70 defenses won't be walling too many people (especially with that psychic typing), and it's speed and attack stats, while certainly nice at 125/125 attacks and 115 speed, it isn't the top of the meta. It's psychic typing gives it a STAB that is resisted by many OU Pokes such as Aegislash. It can't hit steels neutrally, which is a big con with Ferrothron and Skarmory running around. It's typing leaves it weak to Shadow Sneak and Sucker Punch. However, while I doubt it can sweep a majority of the metagame, it isn't useless. After a Nasty Plot, you usually can 2HKO max defense Eviolite Blissey with Psyshock. It has great coverage moves in Shadow Ball, Thunderbolt, Fire Blast, Signal Beam, Energy Ball, Dazzling Gleam, and Thunderbolt, and Hidden Power in addition to it's various psychic type STAB moves.
Many common OU threats not limited to Aegislash Shield (Fire Blast), (Mega) Scizor (Fire Blast), Skarmory (Fire Blast), (Mega) Alakazam (Shadow Ball), Mega Charizard X (Psychic), Mega Charizard Y (Thunderbolt), (Mega) Pinsir (Fire Blast), Azumarill, (Thunderbolt) Thundurus I (Psyshock), Talonflame (Thunderbolt), Gyarados (Thunderbolt), and (Mega) Venusaur (Psyshock) are OHKOd by a Timid +2 special attack from Azelf. These don't even incorporate items, so you can hold things like Focus Sash. Honorable mentions to the list are Mega Gyarados (50% to OHKO with Thunderbolt), Chansey (47.8% with psyshock minimum, assuming max defense, Eviolite, etc.), and Blissey (guaranteed 2HKO with Psyshock). Energy Ball even gets rid of Unaware Quagsire for you. I haven't even mentioned all the hidden powers it can run, such as Ice vs Garchomp and Dragonite and fighting vs Tyranitar. Of course, Azelf will not be able to walk on top of all of these Pokemon, due to things like Prankster Thunder Wave from Thundurus and priority Brave Bird from Talonflame. I am just illustrating Azelf's power to OHKO premier offensive and defensive Pokemon. Not as common, but certainly possible ways to boost, are Charge Beam and Power-Up Punch.
Ironically, despite Azelf's great's sweeping potential, I think it serves better as a lead. Moves like Fire Blast (and others) let you shred lead Pokemon such as Ferrothorn and Klefki (if swag play doesn't get to you first). Stealth Rock is another great reason to use Azelf, bolstered by that great speed. Taunt and Magic Coat let you stop other hazard setters. I think here is good time to review Azelf's great physical moves, which have great utility. The coveted Knock Off is always awesome, U-turn gives more momentum, Zen Headbutt for flinching stab, Explosion (hehehe: useful to at least deal some damage once your utility's up) and great coverage with the elemental punches and Acrobatics. Don't worry, Azelf has more support than just Taunt, Magic Coat, and Stealth Rock. It gets Thunder Wave, Toxic, Trick, Trick Room (not as useful because of Azelf's high speed, but oh well), Torment, Dual Screens, all weather except Hail, and Imprison. The sheer number of these amazing attacks and support moves are amazing. Even better, it is so underestimated (proven by the fact that is a C ranking), you can achieve great success. Don't underestimate this little pixie, or it could cost you the game, set, and match.
Wait, don't leave just yet, I forgot this.
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 completely eclipsed by a Pokemon in the above ranks.
Perhaps it has problems or is outclassed in nasty plot sweeping, but I do definitely not think it does as a lead. With a focus sash, as well as having one of the fasted taunts around, it should consistently set up rocks and hopefully do something else as well. Like Exploding or using Fire Blast on expecting Ferrothorns, etc. As for being "completely eclipsed", I don't think that is true. The only Pokemon that know Stealth Rock and Taunt and are faster are Mega Aerodactyl and Aerodactyl. Because Mega Aerodactyl is a bit of a waste of a mega slot, I believe the only logical comparison is between Azelf and Aerodactyl.
Aerodactyl may be faster, but here is something extremely worth mentioning: 252+ Speed Azelf outspends 252 Speed Aerodactyl by 2 points. Anyway: In exchange for having five less HP and special defense as well as a bit lower speed, Azelf has 20 More attack, 5 more defense, and 65 more Special attack. The defense isn't too important, but listen to this: 252 Azelf has more attack then 252+ Aerodactyl. Azelf can actually run mixed attacking sets, unlike Aerodactyl, and hits harder from both sides. It has a generally better ability in Levitate, while Aerodactyl's best in that department is Double Edge + Rock Head (no Head Smash for it). Aerodactyl's best boosting option is Hone Claws, while Azelf gets Nasty Plot (and Power-Up Punch and Charge Beam). Aerodactyl's best STAB options are the inaccurate Stone Edge, the fairly weak Rock Slide, and the pitifully weak Aerial Ace, while Azelf gets Psychic, Psyshock, and Zen Headbutt. Aerodactyl has decent support that Azelf doesn't in Whirlwind and Tailwind, but it lacks much of the options that Azelf has (such as Thunder
Wave), and it's poor defenses make it an inadequate user of the former in my opinion. Any comments, just reply, and feel free to snip this post.
Also forgot:
The reason I didn't mention or even check is Pokemon were faster, knew taunt, and knew an entry hazard that wasn't Stealth Rock is because of a poll I posted on an RMT. All entry hazards were options, and people had to pick which was best. Out of 20 votes, 18 were cast for Stealth Rock, and 2 were cast for Sticky Web. Spikes and Toxic Spikes each had zero votes. Also, upon voicing the question in the Pokemon Showdown OU chat room, three people responded to the same question, all of them choosing Stealth Rock. In case you are interested, the only Sticky Web-Taunters are Kricketune and Smeargle. The former is terrible, the latter usually uses moves like Spore, and both are far slower than Azelf. Greninja is faster and has spikes, toxic spikes, and taunt, but it has better things to do than taunting, like attacking, and I have never seen a Taunt Greninja. I think that's finally all.