Lucario

Despite Mega Lucario's terrible bulk, it makes up for it with its nice offensive typing, decent Speed, and high Attack and Special Attack stats coupled with the ability Adaptability ,which allows Mega Lucario to deal a ton of damage, even against Pokemon that resist its STAB moves, and OHKO a lot of common Pokemon in the metagame such as Heatran, Bisharp, Terrakion, and Mega Kangaskhan. Lucario is also a really great check to Mega Kangaskhan thanks to Lucario having Inner Focus, which makes it, pre-Mega Evolution, "immune" to Mega Kangaskhan's Fake Outs. Lucario, however, is easily walled by common Pokemon such as Aegislash and Thundurus, and it faces competition from Terrakion and Virizion. Unlike Terrakion and Virizion, though, Lucario can run a mixed or special set which makes it less crippled by Intimidate.