I usually use 88% Lactic Acid. As a second choice I would pick Phosphoric Acid (usually comes as 10%). If you can find a stronger phosphoric acid then it's probably the better choice as I think it has a lower taste threshold than lactic but it's really fairly negligible between the 2. Citric acid has an extremely noticeable taste (think sour candy like warheads) and carbonic acid is a weak acid which is usually found in beer as the result of dissolved CO2.