What is the weakest acid?

Weak acid: Weak acids are those acids that do not completely dissociate or partially dissociate into their constituent ions.

Example: Acetic acid$\left({\mathrm{CH}}_3\mathrm{COOH}\right)$, Phosphoric acid$\left({\mathrm{H}}_3{\mathrm{PO}}_4\right)$, Carbonic acid$\left({\mathrm{H}}_2{\mathrm{CO}}_3\right)$

The dissociation of acetic acid can be represented as:

$\underset{\mathrm{Acetic}\mathrm{acid}}{{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)}\rightleftharpoons \underset{\mathrm{Acetate}\mathrm{ion}}{{\mathrm{CH}}_3{\mathrm{COO}}^{-}\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrogen}\mathrm{ion}}{{\mathrm{H}}^{+}\left(\mathrm{aq}\right)}$