Mega Slurpuff
Typing: Same
Abillity: Sweet Veil/Unburden --> Sheer Force
Stats: 82/80/86/85/75/72 --> 82/115/96/125/85/92
New Moves: Moonblast, Zen Headbutt
Mega Slurpuff is a quite nice offensive pokemon. It can run a physical belly drum set, a Calm Mind + 3 Attacks set, and an all around special attacking set, or even mixed. Its got an excellent ability to boost the power of both its STAB and some of its coverage. While this may all seem to be quite broken, its not really as Slurpuff is not exactly in the best speed tier in the world, also its Belly Drum set relies on innacurate moves. While Mega Slurpuff obviously has its downsides, it is definitely able to make up for them.
Mega Malamar
Typing: Dark/Psychic --> Dark/Fighting
Ability: Suction Cups/Contrary/Infiltrator --> Contrary
Stats: 86/92/88/68/75/73 --> 86/122/108/83/90/93
New Moves: Close Combat
Mega Malamar can now preform the role that regular Malamar never could, as a Contrary Sweeper. It has powerful STAB in the form of both Close Combat and Superpower which both raise its stats in both great and different ways, both increase its defense stat while Superpower increases its Attack and Close Combat increases its Special Defense stat. Both of these can be used differently, Superpower can be used on the more pure offensive sets and Close Combat can be used on more Bulky set.
Mega Barbaracle
Typing: Same
Ability: Sniper/Tough Claws/Pickpocket --> Compoundeyes
Stats: 72/105/115/54/86/68 --> 72/145/130/54/101/98
Mega Barbaracle has not really changed to much from its base form, its mainly just a stat increase. However, the other thing it has changed its ability, and oh boy does this help it out. Theoretically, Mega Barbaracle is now the only Pokemon with a Stone Edge with the base accuracy of 100%, top that on another base 100% accurate move in the form of Cross Chop and you got a Pokemon with amazing stab and coverage, and you got a great Pokemon. Its Shell Smash set is also quite nice, with very powerful coverage moves, pretty slow before it mega evolves, and quite bulky after. Its still got a god awful typing, although it definitely can make up for it.