Hi! I run a rainbow team, and I have a few pokemon I like to switch in and out on it depending on the strength of those pokemon or opposing teams:
I enjoy pokemon like Alolan Ninetales, Froslass, or Vanilluxe because rainbow enables Aurora Veil,
I like cosmic power stored power Clefable (or any pokemon that can run that combo) because of the +2 +2 defenses that rainbow can offer,
Mismagius with Magical Leaf and Mystical Fire, typical Roserade set, and Marvel Scale Milotic are also pokemon I really enjoy on this team,
Cloud Nine Altaria and Mega Altaria is a very powerful choice, especially if you tech in the ability to quickly swap over to misty terrain to empower pixelate. This also powers up Telepathy Gardevoir and Mega Gardevoir, and then allows her and Delphox to make use of Psychic Terrain specifically.
Delphox and Primarina are good "Fire + Water pledge" users, with Primarina having Misty Terrain as well and holding an amplified rock.
You could of course switch to a more grassy sun rainbow team, and my recommendations for that would be Bellossom, Lilligant, Cherrim, Roserade, Florges, and Comfey!