I'm going to respond to people who think being countered by the blobs is not a big deal. Let's say Team 1 picks Gengar and Team 2 picks Chansey to counter it. At that point, Gengar is almost useless to Team 1 because Chansey will just switch in every single time Gengar comes in on something it can threaten. Chansey can then use Wish to seize momentum for Team 2 while Gengar provides no useful team support for Team 1. Team 1 picking Gothitelle to beat Chansey will just compound Team 1's problems. For example, if Gothitelle uses a standard Trick Specs / Scarf set to beat Chansey, then Team 2 could just pick Blissey in response since Goth has only 1 Scarf to Trick away, and then Team 1 would be at a huge disadvantage since it would have almost no useful roles filled (such as SR setter, hazard remover, cleric, etc.) while Team 2 would have some roles filled. I'm just thinking several picks ahead here, and being walled by Chansey / Skarm is a bad place to start for Team 1.
As for Pika Pal's post, these teams don't need to fit any kind of playstyle, they are literally being built to beat each other. Team 2 could use its first 5 picks and defensive Pokemon and its last pick on a sweeper if Team 1 has a huge weakness to one, especially if Team 1 focuses too hard on wallbreaking and not enough on covering its ass against potential sweepers.
The reason I picked Keldeo is that it forces picks which are bad at seizing momentum. Amoongus can't do anything useful for the team except Spore something, but Team 1 could easily pick a grass type to shut it down. Specially defensive Gyarados has to use two moveslots on Rest and Sleep Talk, and it is weak to SR. It is very easy to counter too and gives Team 1 a ton of options to respond. Latias can counter Keldeo, but needs to run HP EVs to take Icy Wind which means it will lack either power or speed which can be exploited by Team 1 (especially if you make Latias the Defogger, then Team 1 can pick a SR setter to beat it). Latios is a very good offensive check to Keldeo, but cannot counter. The point is that a pick like Specs Keldeo forces Team 2 to make tough choices while Gengar and Zard-Y do not. The former just provides the other team with a free cleric / Wish passer while the latter eliminates a lot of options for Team 1 later in the project because it takes up your mega slot.
Mega Metagross would also be a good pick because it similarly forces Team 2 to make a suboptimal pick in hopes of checking / countering it. I mentioned Victini, but it is SR weak and has no reliable recovery and it does not provide much team support.
Given that Team 2 has the final pick, Team 1 needs to apply constant pressure to Team 2 with its picks, otherwise Team 2 will find an obvious weakness to exploit. Going with a pick with an easy counter will force Team 1 to pick in response to Team 2 instead of the other way around and it will be an uphill battle for Team 1.
Edit:
OverlordDerp, I am trying to say that Chansey, Blissey, and Skarmory
are good responses to the Landorus and Gengar sets listed here because they are able to take advantage of free turns by setting up hazards, removing them, passing Wishes, and using Heal Bell. The Gengar set listed here is overspecialized to beat Gliscor and Fairies and thus loses to some of the threats it normally beats with a standard stallbreaker set like Chansey. Landorus loses to Skarmory and Blissey due to lacking Focus Blast. The reason Landorus is so good is that it usually requires a niche Pokemon to counter, but here it is countered by one of the best physical walls in the game and one of the best special walls in the game. Gengar is usually a difficult matchup to deal with defensively because of Taunt and WoW, but this all out attacker LO set allows the other team to just pick something with a good special defense that is not weak to any of its attacks to stop it cold.