The best event was Christmass Tree, but it was one time only. For reccuring events, I would say current Mission Mobilization. If you have someone who will gift you 10 missions every day, then you have 2 free frags daily. After this, 160 coupons per fragment. You c*so wait for Ramen event, she is there for 140 coupons per fragment with possibility to get 1 free frag each day.

Great plates works, too, but they have rigged probabilities. so wouldn't count on them. Just get 20 keys each day and use Draw 10 twice daily. You will get some frags if you are lucky, but don't waste coupons there, I saw people spending thousands and gettin laughable amount of Kushina's frags.