Most of the people says that the Lucky Board is the best Event to spend if you're targeting Power-up items, each time you roll a dice gives you a chance to get various items for Power-up and also for Seal/Summoning Scrolls, while at the same, earning some points too for you to use in Redeeming Items in it's shop, you may choose if you want to redeem Power-up items or Ninja Fragments ..

Same goes for the Lucky Star Wheel, usually 3 items available in that event's Redeem Shop is for Power, Charm Pack, Medium Refines and Myoboku Gifts .. Playing this event is good too since while having some points for you to use on redeeming something, you will also have a chance on hitting the Jackpot Ninja of the Event .. This goes for Wheel of Fortune too, but in this event, you will need to battle for ranking for obtaining additional rewards for participating in this event, the difference between the two events, this wheel of fortune doesn't have a points and redeem shop ..

There's an Event too where you can get a lot of power-up items and you can opt to not spend but still get a lot of items and fragments .. The event is called Great Plates, you can get 20 free keys per day, and with that keys, you can get tons of Mood Scrolls, Myoboku Gifts, Medium Refines and Fragments of featured Ninja of the Event .. In terms of Free Events, Myoboku Trial is also great, you can get Unbound Runes there that you c*e for Cultivating your Summons which gives good amount of power too, at the same time, you c*so get 60 free coupons per day but ofcourse, you must be careful in hitting the totem blocks for you to fini* perfectly and get the free coupons ..

Mobilization Mission Event, Fishing Buddy Event and Some other Events also gives Power-up items .. The only thing for you to do is to choose where and what to spend your resources too .. It depends on your needs or wants .. So be wise and plan on how to spend your resources, at first you may find yourself in some wasteful spendings but as the time passes, you will get the pattern and learn on how to spend good ..

If you are after specific power source and nothing else, then treasure of the sages (usually).

Advnced refines - treasure of the sages.

Deluxe charm material pack (5x50 for 4k) in the shop usually duing charm rebates. If you want specific charm fragments- treasure of the sages and optional charm pack with discount.

Tactics - konoha outlet (225/box).

Medium refines- get them for free from monthly/group shop (takes about a year of getting them for free to pass that level)

medium threads- monthly, sa, space-time redeem section

purification breakthrough pills- top places during events, x500 refine rebate and 8 spins during myoboku wheel (really bad investment, dont do that)

advanced threads- sa, top8 spacetime during fight week, spacetime shop. Currently theres no good event to get them in larger ammount (low power income compared to cost)

cave keys- cave key gift pack in fruit event or in treasure of the sages with discount. Around <800cp is ok price.

myoboku gifts- treasure of the sages, konoha outlet (not sure about that one).

Mood scrolls- 10/1 in the treasure of the sages, optionally going wheels and getting some of them (see below)

Essences- this one is tricky, as it usually requires you to spend a lot. Very good power source, but expensive as heck. You can get it by spending 12k on ability scrolls (DO NOT DO THAT) or in various events for being 1st place. Imo just get them from cave key events- its safe way and you upgrade other stuff as well.

costumes- usually piece them for free and just finish with cp when you are close. Optionally ask on forum for specific costume.

If you are looking for power sources but as a mix of everything, pretty much what Qv said. Also explanation- lucky board is considered the best event for spending because of item value that you get. It loses some value after you finish medium thread/refine level.

Also personal biased opinion: dont buy medium refines/threads, get them over time and buy charms and advanced refines instead. Advanced threads are not worth buying(20cp:1 power ratio which is horrible). Myoboku gifts, mood scrolls and unboud runes are the best to obtain from wheels or great plates.

And dont forget- if you want to spend for power at events with redeemable part (mission dealer, konoha tree, lucky board, lucky stars etc.) always look first at the redeemable part and make sure you want something from there.

thanks guys!

If you are low power then the best answer is lucky board. If you are high power the best options are konoha outlet if you are able to trade a good quantity of scratch cards (do not undervalues those 4k packs... ) with friends, great tree if you are able to reach level 6 or treasure of the sage/daily activity if you are able to get 30%+ discounts.