R Gachas most likely. Since it's easy to get daily ones from missions by doing the chance mission, is the most renewable, and the least important resource for events.

If you have any excess Peronamin/Pudding then you could use those as well to upgrade your prize pool. Overall if you can get the same amount of Rs and SRs with N gachas then you can with R gachas those are the cheapest and possibly the most expendable.

If you want to upgrade your prize pool then you'd have to use Card Reveals and junk like that as well but depending on your level and how well you do on events, R gachas are probably the best option for you.

I'd use just one R gacha at a time but most people use more. Does it upgrade your prize pool? Well kind of but not in a way that you'll benefit by just feeding one R gacha. You can combine one different important item with lots of R gachas or the other way around or simply just 1 R gacha + (insert important item here) if you're looking for better rewards but proceed with care since you might need the important stuff when events come by. I'd read the recipes and try and mix and match if you have excess stuff you don't need, if not 1 R gacha is the best option.