Depends what bonuses you are after and how much you are willing to recharge at once (or during 1 week). Keep in mind that recharging even high ammounts during shinobi east or slot machine doesnt guarantee you getting jackpot ninja.

If you want to recharge not more than 5k, shinobi feast or slot machine is propably your best bet. Shinobi feast will get you 1-3 itachi frags for 5k, as well as some coupons. Slot machine gets you equivalent of shinobi feast value but in items.

If you want to recharge less, about 2100 then platinum recharge is your best friend. Platinum recharge requires you to do the recharge 7 days in a row, so its either 350, 2100 (or 3500 depending on a week) or 21k.

If you want a full ninja and you are willing to recharge higher ammounts (20-40k)- total recharge rebate or 3rd recharge threshold in platinum recharge is the way to go.

As far as pure coupons go, event with red envelopes is the best one (20% of recharged ingots as coupons + additional 10% to send to your group- you can do it duo with a groupmate who's recharging as well so you both get 30% in the end) but its extremely rare- was here only 2 or 3 times.

Ino flowers is garbage, not even worth mentioning.