Rikku- Atk +30%/Res +50% (stacks with almost everything)
Note about this one, it'd be more correct to say this stacks with almost nothing.
When this Soul Break first came out, users thought it would be great for its stacking potential. But it wasn't before long that users realized this is perhaps one of the only cases where it gives two individual buffs.
More plainly, it grants +30% ATK as one buff and +50% RES as another buff. So for example, Shoult would replace the ATK buff but not the RES buff. This is because the ATK and RES buffs are two different buffs with two basic IDs, they're not merged into one.