Highest enol content will be shown by :

The extent of enolization depends on the stability of the enol form. Enol content increases when the enol is stabilized by conjugation, intramolecular hydrogen bonding, or aromaticity.

In 1,3-cyclohexanedione, enolization produces a conjugated system stabilized by intramolecular hydrogen bonding. Hence, it has a significant enol content.

In 1,4-cyclohexanedione, enolization does not generate an extended conjugated system and lacks substantial stabilization. Therefore, its enol content is relatively low.

In 1,3,5-cyclohexanetrione, complete enolization converts the molecule into 1,3,5-trihydroxybenzene (phloroglucinol). The resulting structure contains a fully conjugated aromatic benzene ring.

The aromatic enol form is exceptionally stable due to aromatic resonance stabilization, which is much greater than the stabilization provided by simple conjugation or intramolecular hydrogen bonding. As a result, the keto-enol equilibrium is shifted strongly toward the enol form.

Therefore, 1,3,5-cyclohexanetrione exhibits the highest enol content among the given compounds.

Hence, the correct answer is $$\text{1,3,5-cyclohexanetrione}$$