Throwing in a R gacha by itself is definitely the most worth it because more times then not it will give you a SR Wild Card 3 with a small chance of getting either the Limited N or R. This is especially true if you already have most of the cards that are already in the gacha (increasing a SR by 3 levels is more worth then the unlikely chance of getting a new card or hitting the same card in a gacha and increasing its level by 1 instead).

Otherwise you want to throw in 3 peronamine or Pero Puddings (any combination) in for a chance at the limited SR. once you have it then you don't need to go for it again (unless you REALLY wanted SR Wild Card 5s, which in case is not worth it).