It depends on whether you are playing Sword or Shield, since the fourth and sixth gyms use different typings: Fighting and Rock for the former and Ghost and Ice for the latter.
If you're playing Sword, I suggest selecting Sobble as your starter. The Sobble line only has two major battles to worry about, that being Milo and Eternatus, while also being effective against Kabu, Gordie, Raihan with Ice Beam except Duraludon, and Leon's entire team aside from Rillaboom.
As for Shield, I'd rather suggest Scorbunny. The Scorbunny line struggles with Nessa, Eternatus, Gordie if you're playing Sword, despite its Fighting-type coverage, and Raihan, but bulldozes through Milo, Opal with Iron Head, Melony aside from her G-Max Lapras and Eiscue to an extent, due to its Ice Face ability, Piers with Fighting-type moves and Chairman Rose's Steel-types.
For the Grookey line, though, I would only recommend them if you're looking for a challenge, since Rillaboom, despite being effective against the first two gyms, Gordie, if you're playing Sword and Raihan to an extent, starts to struggle more as you progress further into the game, because its five weaknesses become more subceptible later on and makes Champion Leon more of a challenge, due to having two Fire-types instead of just one.
Overall, I would say Inteleon is the most useful, followed by Cinderace, and finally Rillaboom.