Hello!
Vinegar would be most effective in dissolving the deposits of Hard Water.
Vinegar (CH₃COOH) is a weak acid, while Calcium Carbonate (CaCO₃), being the salt of a weak acid, will be a weak base. These two compounds will react together in the following way:
2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂CO₃
H₂CO₃ reacts further to yield Water and CO₂
H₂CO₃ → H₂O + CO₂(g)
So, vinegar would be effective at dissolving CaCO₃, and we should see bubbles as a proof that a reaction has occurred.
Have a nice day!