Exists to the greatest extent in its enol form at equilibrium?

Enol content depends on stabilisation of the enol. In pentane-2,4-dione the enol is stabilised by a conjugated $\mathrm{C{=}C{-}C{=}O}$ system and a strong intramolecular hydrogen bond forming a six-membered chelate ring; a second ketone group is also more electron-withdrawing than an ester, so its enol content ($\approx 80\%$) exceeds that of ethyl acetoacetate ($\approx 8\%$). Simple carbonyl compounds such as propanone ($\approx 10^{-6}\%$) and ethanal have negligible enol content.