Anyways, Gengar is not S because of its offensive potential or any of the common critera matching it, Mega Gengar is S rank because it is unrivaled in its ability to trap and kill almost any pokemon that can cause your team problems. Palkia is stopping Kyogre sweeping? Gengar gets rid of it. Lugia or Scizor is stopping Xerneas sweeping? Gengar will get rid of it. Theres a support Arceus preventing you stacking hazards? Gengar will remove it. Need a way to put a hole in a stall team? Gengar does it no problem. I could go on. Mega Gengar is, alongside Xerneas, one of the most metagame defining things to happen to Ubers since Arceus itself was introduced - many players dislike XY Ubers purely because of Mega Gengar. Stall teams need a pursuit trapper almost certainly to stop Gengar wrecking the team. Certain pokemon have fell completely out of viability or have had their viability reduced purely because Mega Gengar exists. If there was a tier above S I would expect Gengar to be there. Saying Gengar is A+ is absolute madness.
Gengar do fit on pretty much every team, can often remove pokes that would annoy you, or stop your sweep, but there's still some things that hold him back. Gengar first of all have a base speed stat of 110, which he will keep the turn he mega evolves, and a lot of big threats that you're supposed to trap, outspeed and kill, can kill you before you get your Destiny Bond up. Shadow Tag, just like your speed, won't activate before the turn after you've mega'ed. While Gengar can often force switches, sometimes he can't and either won't have the bulk to safely mega, or he never gets the chance until he's up against a +2 E-Killler who can outspeed and kill. GeoXern can also run Psyshock to laugh at you. Good offensive pressure can stop Gengar. Something that really frustrates me is that on a Sticky Web team, he won't outspeed +2 Xern (especially when they have Psyshock), before he mega evolves, and against good HO builds, you're gonna have troubble finding a free turn to mega. You can always run protect, which I've seen been used and is a nice way to bait pokes who outspeed regular Gengar to stay in, but it is really situasional, and gives Gengar a 4MMS.
Against stall, I've often heard people just syaing that he can doubble into a Chansey, or Blissey, or most Fairy-Types for that matter, and then mega. But you'll have to do this one more time before you can succesfully trap anything. And remember that the one having Mega Gengar is not the only one who can predict. Imo, the Gengar doubble switch is the easiest doubble switch to predict, while not really a reliable way to deal with it, it will still annoy the shit out of Gengar. Stall can also have Puruit trappers who will trap the trapper. Whil this is kinda the same as using Aegislash or Scizor for Xern, one could say that it is not a reason to say that Gengar is less viable, and is used because Gengar is so good. But at the other hand, most of you guys seem to like trapping so insanely much that it might as well still be a good point, imo. We can also compare Gengar and Darkrai (more like the Darkrai we knew in gen5, tho), they're big threats, but most (good) teams has ways to deal with them. Let's also compare him to Landorus-T, he is a nice
one time check to some big threats, but when another threat comes, he isn't gonna help much more. Gengar can "remove" one threat, but chances are another one who is just as threatning to the rest of your team may come, and when your one time check is gone, you might not have much else to defend yourself agaisnt it.
The last thing I wanna say is
that it is not that easy to trap something with Gengar. You mentioned a Palkia can stop your Kyogre sweep, so you just trap him. Well, what if he carries Toxic, or T-Wave? Suddenly you are in a tricky 50-50 situasion at best (if it was Toxic) After that you can either Taunt, and risk getting OHKO'd (Leftovers Palkia don't have that high chance of OHKO'ing, but I'll assume that you used Kyogre to bait the Palkia in), or you could Destiny Bond Hoping he attacks and kills, or you attack twice, hopipng he doesn't attack or KO you the first turn. And to top it off, Palkia can just switch out the first time he comes in. Yveltal can do similar things, like going for a non-attacking move, and start some tricky predictions, or if he got the Taunt off on you, he can now force you out, if less you hit him really hard or something. But Gengars usually don't go for an attacking move when they can't OHKO, and they risk being OHKO'dd themself.
Gengar can use other coverage moves like HP Fire or Icy Wind to remove certain threats, but it makes him lose out on other much needed coverage. The item Shed Shell for Blissey is growing more popular (even tho I haven't run into any)
While I am not neccerarily saying that he is A+ instead of S, I just really wanted to say that Mega Gengar is overhyped. he can not just trap something and then be happy. One of the really good things I like about him is his Toxic, and his two types-immunity, which gives him a lot of free switches. Just stop saying that he has an easy time trapping, because he don't.