The rarest cards are always rainbow rares and secret rares (besides those golden cards in some expansions, but there aren’t any in this one). I know this from experience. And as to which one is rarer, look at a quote from this Reddit post.
“I think you are confusing the names associated with these rarer cards. A secret rare card is any card that is labeled higher than the set number goes. IE if there is 100 cards in the set, card 101 is considered a secret rare. So essentially a rainbow rare card is a secret rare card so their drop rates should be the same, usually 1-2 per box. What you may be referring to as being 1 card per case could possibly be these special gold cards ie solgaleo/lunala, zamazenta/zacian. They are also secret rares but more rare than usual secret rares”
So Rainbow rares are secret rares, so they actually have the same rarity. So, the rarest cards would be all the rainbow rares and secret rares.