Hey,

Personally I would say Itachi Uchiha [Edo Tensei]. He is in many events so collecting free fragments over time shouldn't be a problem.
And about stacking coupons over time and use it in 1 go is always a smart decision, when Fuku Deal's is up.

However even if you collected 20k coupons, and want to use it all on lucky board, for the ninja or power items isn't a bad choice, as a Fuku Synergy most of the time have a synergy with events that aren't really popular to spend on.

This is a great team to run if you have Itachi Uchiha [Edo Tensei]