The most useful treasure by far..is the Kage treasure.

GNW got 2 good Ninjas (other than the super rares): Jinpachi and Edo Sasori.

If you are f2p, forget the savin up for 300 scrolls. Yes it is the optimal strategy, but since you will not win a lot in all the other events, because you are F2P, the biggest excitment comes from getting new ninjas.

Focus on the 5kage Treasure, as in that treasure you will get plenty of useful barriere ninjas and most of the ninjas you can actually use, even later on, in a team.

Really...the only bad ninja in that treasure is Chojuro - maybe Mifune but he is still more useful in lightning than most of the GNW epic ninjas.