Best possible event to spend is probably Lucky Board during Fuku deals. Best possible outcome can come from Lucky Stars during Fuku deals IF you can get the jackpot ninja.

As spending choices go, you can go quite far without any event ninjas if you outpower your enemies. I would suggest spending coupons during Fuku Deals into Training Potions if they are available, and gather event ninjas from Fuku deals alone. You won't be able to get SOSP Naruto this way most likely, but recently there was Angel Konan for 25k, which wasn't a bad price at all.

Issue with the method above is availability of Training Potions during Fuku Deals. If there's not enough potions/command flags to redeem that week, it may be hard to find equally good place to invest into power.

EDIT

While what MonkeyLord83 said is definitely true, I am pretty sure tha Ninja Assist feature is by far the best one to spend into, unless you have all cards unlocked and at level 7+. Also with "6.0" they announced 3 new cards for the feature, 2 of them yet to be released, so saving command flags and training potions for them is not a bad idea. In CN there's 15 cards total I think.