Either lucky board or great tree. Both give lots of items in return for your spending. If you're planning to spend a lot of coups/ingots, do great tree. A medium amount, do lucky board. RNG wheels aren't good for power at all, and mission boxes the return just isn't as high as you only get the one item inside (which granted can be l4 magas, but sometimes it's also crap)

Lucky board and mission boxes are the number one that can give you more power you can redeem a lot just completing missions. Rng wheels will just destroy your life

If you plan to hoard 24k+ coupons the reply is great tree. If you plan to stop at around 10k the reply is a good event with fukurokumaru deal. For example the scratch event was good together with it.

RNG Wheels has its own good benefits also, like for example, the Lucky Star Wheel, if you're targeting some power-up items in the redeem shop of that event then the wheel is good to spend, while having stars to redeems some power-up items, you will also have a chance to hit the jackpot ninja .. But on the other hand, if you targets the Jackpot Ninja then that's the time that you will destroy your life as you say .. RNG is not your best buddy to rely on, so it is much better to keep and save up your coupons and go for the events where you can piece some frags for your liked ninja ..