Kangaskhan reappears as top dog in the current Special Battle season. Ending July 14, this season brings out a new style of battle featured in "Pokemon X" and "Pokemon Y", the inverse battle. In this ruleset, all type matchup are reversed, meaning that, for example, a Brave Bird from Talonflame that normally wouldn't touch an Ampharos will instead tear it apart. This hasn't impacted Pokemon selection very much, though two unusual Pokemon have managed to creep their way up the ladder to place in the top 12.
Consistent playability and easy matchups have afforded the privilege of 1st, 2nd, and 3rd place to Kangaskhan, Gengar, and Breloom, respectively. However, Chansey and Avalugg are new on this list. These two probably enjoy the inverse battle, as Avalugg now resists Fire, Rock, Fighting, and Steel-type moves, and Chansey's only weakness, Fighting, is now only a quarter as effective. The normally terrifying Talonflame is weak to Fighting, Fire, Grass, Bug, and Steel-type attacks, which has resulted in the bird Pokemon dropping all the way down to 10th. Greninja trails at 12th behind Chansey for now.
If you're trying to counter these Pokemon, you might want to bring a Mega Venusaur. Under inverse battle rules, it gains a good matchup against many attacks commonly employed by these Pokemon, including Psychic, Ice Beam, Blizzard, Flare Blitz, and Brave Bird, while still maintaining a neutral matchup against Sludge Bomb and Sludge Wave. You'll need to watch out for the Grass and Fairy-type moves, though, as Venusaur carries a weakness to both.