Sceptile

At first glance, Mega Sceptile may not seem very impressive. For starters, it has bad bulk and a typing that leaves it vulnerable to various priority users including Talonflame, Mamoswine, Weavile, Bisharp, and Mega Scizor. Furthermore, its base 145 Special Attack is relatively average considering Mega Sceptile has to go without a boosting item. Combine this with its lack of a powerful and reliable STAB move, and it's easy to see how most defensive teams don't struggle to handle it. However, Mega Sceptile's niche in OU comes from its ability to handle offensive teams. Its base 145 Speed makes it the fastest Dragon-type available, which is immensely useful in a tier with numerous threatening Dragon-types such as Latios, Latias, Garchomp, and Mega Charizard X, as Mega Sceptile is able to revenge kill all of them, barring Mega Altaria. Additionally, despite Mega Sceptile's relatively average Special Attack, it has access to Leaf Storm, which, thanks to its high Base Power, allows Mega Sceptile to revenge kill frailer threats such as Mega Lopunny. Its Grass STAB move of choice, along with its four times resistance to Water, allows Mega Sceptile to check all of the various bulky Water-type Pokemon in OU, including Slowbro, Manaphy, and Rotom-W. Finally, thanks to its access to Lightning Rod, Mega Sceptile is able to check most of the various Electric-types in OU, including Rotom-W, Mega Manectric, and Thundurus, the latter of which cannot even paralyze Mega Sceptile.