There are two key elements that contribute to my support of the Gengar ban.
First, Gengar is the best mon in the tier.
Second, Gengar speed ties and ohko's itself, and the tie is effectively forced.
An overview on how Gengar works as a pokemon should come first, as, as checkmater pointed out, few people have explained why Gengar is good now, when it wasn't before.
Gengar is only good in a meta where nothing can deal damage to it. This is the key distinction between this meta and previous metas, and i'll point to other examples to illustrate this point.
In VGC '15 and ORAS DOU, Gengar was a pretty mediocre Pokemon. the main reason for this was that, although Gengar has a high special attack stat and decent offensive typing, there were just too many Pokemon that didn't really mind getting trapped by Gengar. Heatran, Cresselia, Scarf Landorus, Thundurus, Amoonguss, Talonflame, etc. can all pressure Gengar effectively. These Pokemon also happened to be extremely high in usage in '15 and ORAS DOU. In a meta where every Pokemon is a threat and Gengar doesn't have a chance to effectively trap and support its team, Gengar can't do much. Gengar is unable to pressure any of these Pokemon with a KO, and all of them deal a bunch of damage back to it.
VGC '16, however, saw the rise of Gengar to prominence, securing 3 of the top 4 placements at Worlds. Why was Gengar good in '16, when it hadn't been good in '15? The reason was that, with the use of pokemon like Kyogre, Gengar could beat the only Pokemon in the format that could deal damage to it, Groudon. Gengar was able to pressure Groudon w/ the rest of its team, and then Kangaskhan, Smeargle, Cress w/o Psychic, Bronzong, Xerneas, Kyogre trapped in the sun, etc. couldn't deal with it. Gengar could then use its solid special attack stat to deal good damage w/ Sludge Bomb and control the board for its partners, which could also now deal more damage safely.
The key to Gengar is twofold: 1) Don't let it take damage and 2) have a way to blow up things.
The current meta allows Gengar players to fulfill those two maxims. First, Gen 7 introduced Z moves, allowing Gengar users to burst OHKO threats, like bulky waters and Landorus w/ Kartana's Z-Leaf Blade, Incineroar with Fightinium Z Pheromosa, as EmbC showed us recently, or every Pokemon with Landorus's Z-Earthquake. Second, Gen 7 (eventually) introduced Incineroar, which offers a bunch of buffs to Gengar.
First, Incineroar offers role compression that Gengar sorely needed. It gives: damage reduction, a way to remove berries, Fake Out for extra control, fire typing to hit Steels, U-turn to help abuse the trap as much as possible, and a safe switch out for Gengar.
Second, Incineroar is also very tough for offensive teams to break, shifting the meta towards a bulkier offense, which is easier for Gengar to take advantage of (and even if Gengar isn't great in a matchup, Incineroar can put in enough work to cover for it). The FWG core of USUM DOU of Incin, Fini, Kartana/Amoonguss is very weak to Gengar, even as they are strong in most matchups. Gengar can hit Incin for a very solid ~40% (with a 30% poison chance) and can avoid the OHKO from any of Incineroar's moves. It can also just switch in its own Incineroar either on the Gengar slot, or next to it to set up a Fake Out and reduce the opposing Incin's damage. Gengar does 90% to Fini at neutral, so if Fini gets chipped, it can get trapped and KO'd easily. Kartana doesn't carry moves that do damage to Gengar and has to run at least 176 spdef investment to take a Shadow Ball from Gengar.
So Incineroar singlehandedly creates the 2 conditions that are necessary for Gengar to have success, in addition to Fairy types being quite common, which gives Gengar things to do on its own. However, this doesn't explain why Gengar is bannable, it merely explains why it was good.
If this was the only problem with Gengar, I'm not sure I'd be willing to ban it. Gengar is good, but it's possible to play around it by getting Calm Minds up with your Fini, setting up Tailwind for your Zygarde, positioning Scarf Lele/Landorus well, using Incin well, etc. However, the biggest problem, in my opinion, is that Gengar being so good makes its speed tie with itself an unhealthy force in the metagame.
On speed ties: most pokemon don't beat themselves. Pokemon that invest in speed tend to do so for other mons in the metagame, and the speed tie is just an added bonus. With Gengar, however, i invest to max speed timid exclusively for the speed tie, because of how important i've found it is in the Gengar mirrors. Contra Bowman, and in line with EmbC's experience, I've found that the Gengar speed tie is often an unavoidable part of Gengar mirrors. This is for a couple reasons.
First, someone who is slightly down in position might feel that they have to go for it to win. This means that players who are down play more riskily and try to force the position to a ~50/50 to win as opposed to a far higher chance to lose. For reference in games, check my post
here, and the replays linked. In my game against Emforbes, I was slightly behind and had to go for the tie to win. In my game against Bowman, I was significantly ahead and he had to go for the tie to win.
This wouldn't be a problem exactly, if not going for the tie wasn't so punishing, which is the reason the tie is unavoidable. For, say, M-mence speed ties where both have Draco Meteor, there are often easy switches (steels, fairy types) and you can burn the special attack and recover positioning easily. With Gengar speed ties, not going for them also risks losing the game, by dint of losing positioning so hard that you would have had better odds to win the game by going for the tie. Consider the game I linked vs bowman in my earlier post. I *could* have switched out my Gengar to Incineroar rather than going for the tie, but in so doing, I get extra damage on the Incineroar and barely improve my position; indeed, Bowman could probably force the exact same 50/50, or get better odds to win. In my game vs Emilio, he had no choice, cause none of his remaining switch ins resisted Ghost. Even more critically, if you choose not to go for the tie, your switch in will get chunked
and trapped. Most of the answers to Gengar work as answers to it because of its status as a glass cannon; Landorus and Zygarde beat it because Gengar can't OHKO, and they can KO back. However, that math changes significantly when they take damage on the switch in. Incineroar also doesn't work cause it doesn't OHKO back, or might be next to your Gengar.
So Gengar is forced to run max speed to ohko itself, which is a situation that comes up in most high level Gengar mirrors, and winning the tie can be a massive momentum swing. This turns high level Gengar mirror matches, in many ways, into a 50/50, unless one person is blowing the other one out (my match vs Emforbes was a 50/50 because the positions were even up to the speed tie point, my match vs Bowman I was still able to win since I had amassed a decent lead already).
Gengar is a good Pokemon, that needs specific meta conditions to make it valuable; its tools rely on the meta being a specific pace, and not being able to deal damage to Gengar. That's the current meta, so Gengar is very good. I don't think I've used a team with a positive Gengar matchup in my 10 games of Snake, my entire DLT run, and my DLT top 16 match, or seen a team that was positive vs Gengar while maintaining its overall ability to respond to the rest of the metagame. The real
bannable offense that Gengar commits though, is turning high level mirrors into cancer with the speed tie being so critical to the outcome of games. In Gengar mirrors in DOU currently, skill is not the true factor in who wins (although massive outplay can overcome the speed tie).
