Fire attacks cover Gyms like Milo's and Melony's. They're also solid against the Champion Cup, Chairman Rose, and other random Trainers.
Recommended Pokemon: Cinderace, Arcanine, Centiskorch, Chandelure, Salazzle
Fighting attacks are good against Marnie, Gordie's Gym, Melony's Gym, Piers, Chairman Rose, and quite a bit of the Champion Cup.
Recommended Pokemon: Machamp, Pangoro, Lucario, Hawlucha
Ground types deal effectively with Kabu's Gym, Gordie's Gym, Chairman Rose, some of the Champion Cup, and a lot of random Trainers.
Recommended Pokemon: Excadrill, Mamoswine, Whiscash, Sandaconda, Flygon
Flying moves can take out Milo's Gym, Bea's Gym, some Pokemon in the Champion Cup, and is a solid coverage type in general. Corviknight in particular is phenomenal.
Recommended Pokemon: Corviknight, Gyarados, Noivern, Togekiss, Hawlucha
While not the best, it has outstanding Pokemon like Dracovish and Barraskewda. Water moves deal nicely with Kabu's Gym, Gordie's Gym, some of Raihan's Gym, and many other random battles.
Recommended Pokemon: Inteleon, Barraskewda, Crawdaunt, Whiscash, Gyarados, Lapras, Dracovish
It's a very good type to have in the endgame, proving its worth against foes like Eternatus, Raihan, and quite a bit of Leon's team.
Recommended Pokemon: Noivern, Haxorus, Flygon, Duraludon, Dracovish, Dragapult
Fairy has always been a good type. It can take out Bea's Gym, Piers' Gym, part of Raihan's Gym, some of the Champion Cup, and has tremendous coverage on a lot of random Trainers.
Recommended Pokemon: Gardevoir, Togekiss, Sylveon, Grimmsnarl, Mimikyu
Ghost is a decent type. They hit Allister super effectively, along with some of Leon's team, and Bede's.
Recommended Pokemon: Gengar, Mimikyu, Aegislash, Chandelure
Hope I helped!