Terrakion is a tough case, but I think it belongs to A-. Terrakion was dropped all the way down to B+ at early ORAS, the reasons were obvious, Gliscor was spammed, Landorus-T best set at that time was the defensive rocks variant, Mega Slowbro was seen like everywhere, Mega Gallade was overhyped and it was even proposed to be S, and this hampered Terrakion since it was an offensive check to it, not to say that Greninja outsped it and Landorus could take a hit and retaliate back. Since then, Terrakion became BL, went to B+ really fast and was never brought up again. Now the metagame has changed drastically, Landorus and Greninja are gone, Gliscor's usage isn't that high anymore, Landorus-T's best sets at the moment are the offensive ones, Mega Slowbro hasn't seen much usage as of late and Mega Gallade in the end became BL and in the same rank as Terrakion lol. The metagame since then has changed a lot and it's in Terrakion's favor. Yes, it undeniably has somewhat of a Speed problem, as it's outsped by notorius threats such as Mega Metagross, Mega Diancie, Gengar, Mega Lopunny, etc. But do you know who is A+ and it's also outsped by them? Keldeo. You could argue that Keldeo has a fantastic Scarf set to avoid this kind of problem, but Terrakion does too, and it has high powered STABs to back it up. Yeah Keldeo has Hydro Pump too but it's unreliable for cleaning, and let's be honest here, how many times did you click Scald before Hydro Pump? exactly. Terrakion also doesn't rely on burning things to wallbreak, instead he takes his brutal power and blows something back + isn't Starmie bait to spin your hazards. I'm going to stop the Keldeo vs Terrakion comparison here as they're not really comparable besides them sharing the fighting-type lol, obviously Keldeo is more viable than it thanks to great defensive typing and the ability to invalidate counters thanks to burns. Alright so let's see how many fighting types we have on the S to A- ranks. Two. Mega Lopunny (who takes a mega slot) and Keldeo. Alright, Terrakion might face competitions from other MEGA fighting types which reside in B+. However, there are like 16 megas that aren't fighting types in the S to A- ranks. Most of the time I'd rather have a Terrakion with something like Mega Altaria for example than Mega Heracross and be forced to support it. Let's compare Terrakion with something on A- rank. Volcarona for example has earned a solid spot in the A- ranks, and it's hard not to see why, a lot of teams are 6-0'd by this thing lol. But it needs huge support in hazard removal, and being able to dispatch of his counters. Sometimes from the start Volcarona is just dead weight and won't do anything because there is a pokemon that walls it, say Heatran, so you need to support it with lures unless you can afford to run a Dugtrio lol, and the opponent can see this by the make up of your team so it might not even work. But you know what? Terrakion doesn't need that much support, the only thing you need is probably Keldeo, Mega Metagross switch-ins, which are big threats and everyone should be prepared for them already. Terrakion won't never be deadweight, it always can do something. Proof:
+2 252+ Atk Life Orb Terrakion Stone Edge vs. 252 HP / 252+ Def Mega Slowbro: 218-257 (55.3 - 65.2%) -- guaranteed 2HKO
This is the bulkiest thing you will find to wall Terrakion, and it doesn't usually pack that much defense because it rathers patch up his mediocre special defense, Mega Slowbro's defense is high as it is anyways. This means that Terrakion will be able to do something if you give him a try. Yes Volcarona sweeps certain teams from the start, but if for example you don't have HP Ground and you're not able to lure Heatran you're in trouble.
So in the current meta we have two main playstyles at the moment, Hippowdon balance and Offense. Terrakion is able to lure Hippowdon and other stuff if you get to SD, and that is huge since Hippowdon is used to check a lot of threats and removing it from the match means no rocks for the opponent and the lose of Hippowdon, a big defensive threat to any team. Then we have offense, which Terrakion kinda struggles unless it runs a Choice Scarf. However if you get him safely something is losing atleast 50% to the Life Orb variant. Yup the rise of Tornadus-T, Thundurus and Alakazam annoys it, he can't set-up on every game, that would be just theorymoning how Terrakion can easily blow his counters, and that's not true, he can't set-up in every game, that's a fact. But that's why I'm nominating it to A- rank and not A+ lol. If I don't get him to A- rank atleast let's just discuss it for a bit, he hasn't been brought up by a while, give him a try. By the way, if I'm missing something that really hampers Terrakion and didn't say it, feel free to notice me.