Mega Maw if used by a good player will always take at least one Pokemon down with it, and can take down a whole team if there is no priority on their side, or if the users are gone. One Swords Dance lets it at worst 2HKO everything that doesn't resist Sucker Punch. That and it has good bulk.
Mega Khan can shut down pretty much every Ghost and Psychic type in the Meta with Scrappy Fake Out + Mega Evo Sucker Punch. That, and it has 2 chances to critical hit on each move, insane bulk, good speed, and is almost not revenge kill-able once it gets to +2 (at least not in OU). Mega Gardevoir pales in comparison, as it always loses to common things like M-Metagross, Scizor, Magnezone and Scolipede.
And interesting... Mega Cards that are meh?