What is the heaviest possible water using oxygen and hydrogen isotopes, and what might it's physical properties be?
The heaviest water obtained with stable isotopes would be $\ce{D_2^{ 18}O}$.  This compound is not well-charactetized itself, but Ref. [1] indicates that the melting point is only about 0.28°C above that of ordinary water.  The implication is that overall physical properties of $\ce{D_2^{ 18}O}$ are close to those of heavy water with oxygen-16($\ce{D_2^{ 16}O}$); apart from density the isotopic effect of $\ce{^{18}O}$ versus $\ce{^{16}O}$ is much less than that of deuterium versus protium/&quot;hydrogen&quot;.
References

F. Steckel and S. Shapiro, &quot;Physical Properties of Heavy Oxygen Water.  Part 1.  Density and Thermal Expansion&quot;, Trans. Faraday Soc. 1963, 59, 331-343.