B+ -> A-
Mega Sceptile is something that has flown under the radar a bit. Before Mega Altaria was banned, it sucked because Altaria could just use it as setup bait. However, Mega Sceptile is now amazing. First of all, it has amazing 110/145/145 offenses, allowing it to run a variety of sets and outspeed the majority of the unboosted metagame. The Special Sub set is very good because it can set up a sub on most bulky waters in the tier if they lack Ice Beam. It also has amazing coverage in grass/dragon/fighting. Once Sceptile is behind a Sub, it almost always gets a kill unless the opponent has a specially defensive fairy like Florges. Alternatively, it can run Leaf Storm over Substitute if you want to be able to nuke something on the switch in.
Calcs:
252 SpA Sceptile Giga Drain vs. 252 HP / 0 SpD Suicune: 236-282 (58.4 - 69.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Sceptile Focus Blast vs. 252 HP / 120 SpD Filter Mega Aggron: 222-262 (64.5 - 76.1%) -- guaranteed 2HKO after Stealth Rock
252 SpA Sceptile Dragon Pulse vs. 0 HP / 0 SpD Hydreigon: 330-390 (101.5 - 120%) -- guaranteed OHKO
252 SpA Sceptile Focus Blast vs. 0 HP / 0 SpD Lopunny: 294-346 (108.4 - 127.6%) -- guaranteed OHKO
While the damage output may be a little but underwhelming, the main reason to use this is because it is a fast revenge killer that still packs a punch. It's best paired up with stealth rock/spikes support so it can net some KOs. However, one big issue with Mega Sceptile is that it is walled by SpDef Jirachi. This leads me to another great set, the SD set.
Although it may not be as immediately powerful as the Special set, the SD set is great and it catches a lot of people off guard. Here are some calcs:
+2 252 Atk Sceptile Earthquake vs. 252 HP / 0 Def Jirachi: 388-458 (96 - 113.3%) -- guaranteed OHKO after Stealth Rock (nice switch in m8)
+2 252 Atk Sceptile Earthquake vs. 0 HP / 4 Def Beedrill: 391-460 (144.2 - 169.7%) -- guaranteed OHKO (assuming it is adamant or you win speed tie)
+2 252 Atk Sceptile Leaf Blade vs. 0 HP / 0 Def Lopunny: 303-357 (111.8 - 131.7%) -- guaranteed OHKO
+2 252 Atk Sceptile Dragon Claw vs. 252 HP / 0 Def Celebi: 232-274 (57.4 - 67.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery (this walls the fuck out of the special set)
I would nominate this higher, but it has things holding it back, such as its mediocre defensive typing and bad bulk. Its damage output is not amazing, but it deserves to be higher than B+.
I would put the calcs in spoilers but idk how to do them tbh