A17
Smash Ace
Fully charged Shield Breaker does 23%, and kills Mario at 49% in training from his starting position on FD.
Fully charged f-smash does 21%, and kills from 58%.
So for punishing broken shields, Shield Breaker is your best option.
I've been looking up answers all morning at work. Has it been calculated how much shield damage SB does? Tap? Half? How does it compare to Marth's?