for me, now. In the ranked shop: i only by GNW sakura ( she is bad-ass in ranked battle) after i got GNW sakura 4*, i start buying Sasori ( very good for ranked battle as well). When Sasori is 4*, i will come back to finish off GNW sakura to 5*.

In group shop, i only buy Konan. Konan is decent ninja for ranked battle if you run akatsuki/ tendo/ TAG / combo team like utakata, roshi, hidan both version.

survival shop: i only by kabuto and sometimes, cultivation rune when needed. Kabuto is one of the best healer in the game. If you use water main team since day 1 like me, water main-kabuto-kimimaro-ukon sakon, is the best team for strong approach at any level even expert as well.

Summoning shop, I will get gamaken summon first since it work well with water main, kabuto kimimaro and the last slot :ukon sakon/ kidomaru for strong approach.

For space time shop: get power up item after you get choza. Sasori 100 puppet you can get him from sage naruto treasure.
Seal scroll save it for 5 kage and Jin 1 - 210 pull roshi and Jin 2- 210 pull Han

Coupon: save up to get curse hidan first (~6000 coupons). Curse hidan is your best friend up until 90~100. Curse hidan, roshi will carry you through ninja exam lvl 120 and all the ultimate training. ( I finished ultimate training with water main, curse hidan, roshi and killer bee 7 swords team).

For survival trial, i use water main, kimimaro, either sailor sakura or GNW sakura, and tsunade or kabuto. Sailor sakura is free in lucky board event for around 4000~5000 coupons. After you get roshi. replace kimimaro with roshi. You c*e normal sakura until you have other sakura.

Strong approach: I use all f2p ninja: kimimaro, kidomaru, ukon sakon kabuto.and gamaken summon. with water main, curse marked, chaos passive and mystery: shark boom. i always ended up full hp team after the fight.

in all of the events, i always try to get power up item like refine, unbound rune, summoning scroll for rune, cave key, magatama as much as possible unless they have sailor sakura and curse hidan.

Tips: Tenten is a really good ninja if you like water meta: tenten. I dont like her that much so i dont use her. (team water main, mabui, Tenten and either darui or mei)

Last but not least, just enjoy the game and set goals for yourself, you dont have to compete with p2w players. it takes lots of time to get some ninja but you will ultimately get those ninja anyway. Don't over-spend on luck-based events for rare ninja, it's trap. Personally, i didnt get any rare ninja from those events