guess i'll chime in as well. haven't really bothered to read the whole thread, so i assume the stuff im about to say has already been said
initially, i was underwhelmed by mega gengar. like many people, i saw those offensive stats it has and thought: omg ultimate special sweeper! but i forgot one really important thing. and thats that gengar is a ghost. and ghosts are SNEAKY!
after my incredible revelation, i turned to gengars support movepool, and realized that if one wishes to truly benefit from mega gengar they (hard as it may seem at the beginning) must shy away from base 170 special attack and an excellent special movepool, and instead abuse 130 base speed, a slew of support moves and the best ability in the game.
honestly, perish song gengar is just sick. especially with voltturn support, as it can then come in on its targets for free with very little prediction needed, perish trap them, sub and protect once, and then get away scot free with plenty of hp left to start the cycle all over again. if the opponent doesnt have a pokemon that can outspeed or ohko mega gengar (and if youre smart you won't allow this, since you choose what you're trapping lol) then they're gone. i used a set of perish song, substitute, protect, shadow ball. disable and taunt are of course options over shadow ball, but i still cant bring myself to nto use 170 special attack in some way. its also really useful thanks to the brilliant neutral coverage it has, and for when you dont need gengar to trap something without wasting some of your hp. mega gengar is guaranteed a kill on any pokemon of your choice, and a second is also very likely, because often even if your opponent knows exactly what youre going to do, they have no defense against it. and to me, thats the defintion of an uber.
you need insurance against strong priority of course, such as against brave bird, aqua jet and bullet punch. lol rotom-w is such an amazing partner for every bloody pokemon in this tier. it even has the slow volt switch for mega gengar :')