>>20336505M-Gengar is broken because its relatively forgotten about in favor of lelXerneas and lelKyogre, and has a monstrous Sp. Atk stat with SHADOW TAG.
Why do you think Mega-Alakazam has been dropping in and out of OU, despite having similar stat-spreads to Mega-Gengar? It doesn't have Shadow Tag and is easy to revenge kill.
Mega-Gengar is the most broken Pokemon ever invented because it is impossible to track down. It not only counters Mewtwo, but, with its wide array of moves and high powered hits, counters 75% of Ubers. It is impossible to revenge kill due to being the pivot of the team, with only 2 weaknesses and 3 immunities. You can't stall it due to its wall-breaking potential and immunity to poison. You can't revenge kill it, because it can smell that sucker punch coming from a mile away and just status your Mega-mom or run. You can't set up on it, since it can taunt your faggot ass 6 ways from Sunday.
The only way to defeat Mega Gengar is to predict its switch in and snipe it or try and catch it off-guard by bringing an unusual scarf Pokemon into battle. Remember, it can see how boldly you threw out Kyogre and jerk it over the fact you were retarded enough to be so predictable.
LC is lol, and Aegislash is banned from OU due to Fifty-fifty Fujisaki.