Mega Medicham:

Mega Medicham was a force to be reckoned with in UU. A base 100 Attack stat may not seem much, but when it's boosted by Pure Power, it is unbelievably strong. Mega Medicham's high-powered STAB move in High Jump Kick let it beat out a lot of the most sturdy physical walls in UU as well as some bulky resists. Here are calcs that prove its power:

252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 252+ Def Hippowdon: 228-268 (54.2 - 63.8%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 252+ Def Vaporeon: 348-411 (75 - 88.5%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 252+ Def Suicune: 231-273 (57.1 - 67.5%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 4 Def Mew: 176-207 (43.5 - 51.2%) -- 62.9% chance to 2HKO after Stealth Rock and Leftovers recovery
252 Atk Pure Power Mega Medicham High Jump Kick vs. 4 HP / 0 Def Latias: 193-228 (63.9 - 75.4%) -- guaranteed 2HKO after Leftovers recovery

Furthermore, it had excellent coverage in Zen Headbutt/Psycho Cut, Thunder Punch, Ice Punch, and Bullet Punch. It could opt to keep itself healthy with Drain Punch, and it had the option of safely Mega Evolving with Fake Out. Bullet Punch was another source of priority it that let it beat Fairy-types like Mega Gardevoir without risking a Speed tie. It could even make use of the switches it forced to set up Substitute and become impossible to revenge kill without Infiltrator Pokemon or Choice Scarf Boomburst Exploud. It was because of this that Mega Medicham had almost no safe switch-ins and was almost impossible to counter, or even check, defensively. Mega Medicham's base 100 Speed was also on par with the tier and its decent 65 / 85 / 85 bulk let it survive a surprising amount of non-super effective hits, as well as some powerful STAB attacks, from a wide variety of Pokemon. The only Pokemon that could take on Mega Medicham were Doublade, defensive Reuniclus, Granbull, and defensive Mew. If it lacked Thunder Punch, then Slowbro could also check it. Basically, there were practically only four Pokemon that could reliably switch into Mega Medicham, which just showcases what a monster it really was.