Ok, so I'm getting pretty tired of seeing these points about, "Mega Gengar isn't broken because it can't run everything at once, so there's always something that can still beat it." Arguments like this completely miss the point:
Mega Gengar doesn't need to be able to kill everything, but it can be tailored to kill almost anything. Mega Gengar is not like Keldeo. Keldeo's problem was that it was designed to sweep through teams or do as much damage as possible, and it relied on specific Hidden Power types to beat its usual switch-ins. Mega Gengar, on the other hand, is not meant to sweep through teams or anything like that. It's designed to eliminate checks and counters to its teammates one Pokemon at a time. It's also not like Keldeo in that it's not a matter of it being dangerous until you know its moveset. Even if you know its moveset, it does nothing to solve the problem. With Keldeo, knowing what Hidden Power it had would tell you what was safe to leave in on it or bring it to check it. I could literally bring Mega Gengar in on your Landorus-T, tell you in the chatbox, "Hey, just letting you know that my Gengar has HP Ice and I'm about to kill your Landorus-T with it," and you'd be helpless to stop me.
Several people have this misconception that just because Mega Gengar isn't a mind boggling sweeper like Blaziken or an insane wallbreaker and general hard-hitter like Deoxys-N, it isn't broken. What they fail to understand is that Mega Gengar's sweeping potential and raw power were never the primary issue here. The issue is how it can use its natural talents to guarantee that it kills what it wants to kill. So it can't run Shadow Ball + Sludge Bomb/Wave + Thunderbolt + HP Ice + HP Fire + Focus Blast + Substitute + Perish Song + whatever else. Who cares? Seriously, who cares? If my team is planned around a Thundurus sweep, why on earth would I want to run Thunderbolt for Gyarados or HP Ice for Gliscor when Thundurus can kill those himself? Let's say I'm basing a sweep around RP Landorus-I. Some of the best answers to RP Landorus-I pre-6th Gen were Gyarados, Latias, Celebi, and SpD Rotom-W. All I really need for those three are Shadow Ball, Sludge Wave, and Thunderbolt. Latias is heavily damaged or killed by Shadow Ball, Celebi suffers similarly to Sludge Wave, and Gyarados outright dies to Thunderbolt. Mega Gengar tanks Rotom-W's Hydro Pump pretty comfortably, and while Rotom-W can escape with Volt Switch, it won't get out without taking up to ~50% damage from Sludge Wave, which puts it right into range of Landorus-I's Focus Blast. That's just 3 moveslots; I still have one left for Substitute, HP Fire, or whatever else I want to run. The big deal is that there's nothing you can do about it! Once I bring Mega Gengar into your counter, you're sunk. You're just going to have to take your medicine because there's no counter-playing a Pokemon that can't be switched out of.
Wobbuffet can trap anything gengar can, and encores support mons and countercoats sweepers. I fail to see how this is different than gengar, who has to have different movesets to accomplish those goals. Base stats are a relevant argument, but I'd like to point out 600bst isnt even that great for a mega. Wobbufet comes in on a sweeper and stops it cold. Anything boosted can generally destroy gengar. They have different jobs, really, and should be treated as such.
The difference between Wobbuffet and Mega Gengar is that Wobbuffet is highly dependent on what the opponent actually does. So many things can go wrong when using Wobbuffet if you don't guess correctly. You might Encore the opponent's setup sweeper only to have them hit you directly and then KO the next turn. You might try Counter or Mirror Coat only to give your opponent a free Substitute, get statused, have the opponent set up another boost, or whatever else. Mega Gengar doesn't have this problem. When it comes in on something that its set is designed to kill, it will kill it. This is not a question. This is not a matter of proper prediction. This is a fact. If I want to kill your Slowbro and open up for a Terrakion sweep, I merely have to bring it in and your Slowbro will die to Shadow Ball the next turn.
Having read those posts, I think it's a bit obnoxious to assume that a total of 3 movesets take out 70 pokemon. I can do the same thing with 3 movesets. Listing at least 12 moves to deal with 70 pokemon is a bit insane, I'm sorry.
rey's first list of things beaten by Mega Gengar contained 62 Pokemon. 60 of them were beaten with a simple set of Shadow Ball / Sludge Wave / Focus Blast / Thunderbolt. It is not an issue of using 12 different moves. Even then, needing more than four moveslots to beat everything is honestly not a problem because, again, Mega Gengar is not designed to beat everything. It is tailored to beat only what it needs to beat in order to facilitate a sweep for a teammate, and it can usually do this with no more than 3 or 4 moveslots.
Garchomp is another mon with options (life orb sets, scarf sets, now megachomp sets) where you similarly have to wait and see what it has, but if you guess wrong you can end up losing a mon for free just like Mega Gengar.
There's a huge difference between Garchomp and Mega Gengar: Garchomp doesn't have Shadow Tag. The fact that prediction is even involved when facing a Garchomp makes it far different from facing a Mega Gengar. When Garchomp comes in, you have to predict what it's going to do, but your opponent also has to predict how you're going to respond and play accordingly. Prediction in this case goes both ways. When Mega Gengar comes in, there is very little prediction involved. Your opponent knows good and well that you can't switch out to anything that can actually beat Mega Gengar 1-on-1. They don't have to predict, and no amount of proper prediction is going to save your Pokemon.
Unlike blaziken (which i still disagree with, but can see the logic behind) it can not suddenly and unstoppably end a game. It does not have that kind of power. It CAN with good prediction and play uncontestedly take out 1 pokemon. But so can many others.
As explained earlier in this post, Mega Gengar isn't built to sweep through teams like a Blaziken. It's built to eliminate a specific list of Pokemon with no hope of retaliation so that a teammate can sweep. It does not rely on good prediction, because once Mega Gengar is in on something it can kill, that thing dies. Period. So many other Pokemon
cannot do this because they cannot trap things. Only a small handful of other Pokemon have the ability to trap like Mega Gengar can, and problems with those comparisons have already been addressed ample times before.