Ok, so, Megamax is a format where Gigantimax Pokemon can Mega Evolve. They gain new moves and a +100 gain to their BST's. You can also use normal OU Pokemon, not only Megas. It's played in Regional Dex OU.
Here are the viability rankings:
S Rank
Clefable
Coalossal-Mega
Magearna
Mandibuzz
Toxapex
A+ Rank
Centiskorch-Mega
Cinderace
Duraludon-Mega
Excadrill
Rillaboom
Rotom-Heat
Urshifu
Urshifu-Rapid-Strike-Mega
A Rank
Appletun-Mega
Blissey
Dragapult
Ferrothorn
Hippowdon
Hydreigon
Kommo-o
Sandaconda-Mega
Slowbro
Volcarona
A- Rank
Alakazam
Amoonguss
Corviknight
Drednaw-Mega
Zeraora
B+ Rank
Butterfree Sprite Butterfree-Mega
Chansey
Charizard-Mega-Z
Flapple-Mega
Gastrodon
Kyurem
Marowak-Alola
Rhyperior
Skarmory
Tangrowth
Torkoal
Venusaur
B Rank
Aegislash
Azumarill
Blastoise-Mega-Z
Gengar
Grimmsnarl
Hatterene-Mega
Hawlucha
Jirachi
Magnezone
Mantine
Pikachu-Mega
Quagsire
Reuniclus
Terrakion
Togekiss
B- Rank
Bisharp
Charizard
Conkeldurr
Crawdaunt
Diggersby
Ditto
Grimmsnarl-Mega
Heracross
Keldeo
Kingdra
Kingler-Mega
Mew
Pelipper
Rotom-Wash
Scizor
Seismitoad
Slowking
C Rank
Araquanid
Barraskewda
Chandelure
Cloyster
Copperajah
Copperajah-Mega
Garbodor-Mega
Gyarados
Hatterene
Krookodile
Lapras-Mega
Lycanroc-Dusk
Mamoswine
Necrozma
Ninetales-Alola
Obstagoon
Pincurchin
Porygon2
Raichu-Alola
Salazzle
Sandaconda
Shuckle
Slowbro-Galar
Snorlax
Talonflame
Toxtricity
Tyranitar
Venusaur-Mega-Z
Weavile
Weezing-Galar
The Bad Ones
D Rank: Voted Unviable But Could Be Decent Actually
Alcremie
Snorlax
Blacklist: Totally Unviable Megas
Cinderace
Gengar X
Inteleon
Machamp
Orbeetle
Rillaboom
Toxtricity
Meme Tier: Totally Unviable Megas that due to their nature are probably not going to be buffed
Eevee
Meowth
Source
If you want sample teams, go here.
The list of Megas, their types, abilities, stats, and new moves, this should have them.
Hope this helps! :)
