Unfortunately it all depends. Any one of the 3 'could' make more of a difference than the others. If your power delivery is the main drawback in your system, a new PC could make as much or more difference than an interconnect. Plus, the type of changes are different for each of those types of cables.. AND on top of that, the combination of these various cords may yield better or worse changes.
Each cord is a separate variable and it'd be impossible to say which one would make the most difference in your system. In MY system, Power cords and interconnects make more dramatic changes than speaker cables however, the speaker cables do something special that I can't put my finger on.... and it has to be right to enjoy it.
There's no definitive answer on this one..
-C