Liquids that keeps the lowest temperature for the longer time after being frozen
To maintain a stable temperature while heat is entering a system implies some phase change: that is, the substance is either melting or boiling, keeping the temperature the same, while some part is changed form solid to liquid or form liquid to gas.

Pure water freezes at 0 °C, but if another substance is added, the freezing point (f.p.) may be lowered. Ordinary table salt, $\ce{NaCl}$, can lower the f.p. to -20 °C. Calcium chloride, $\ce{CaCl2}$, is even more effective. Both are used for melting ice from roads, with $\ce{CaCl2}$ preferred in cold weather.

You might test various concentration to see what best meets your needs. Ideally, the solution f.p. should be a bit above the temperature of your freezer.