What is the perfect Ice Cube? I have a freezer with -25°C temperature and Soda with +30°C.
What is the ice-cube-sized material I can put in the freezer that can bring my glas of soda to the lowest temperature?
Are e.g. saltwater icecubes better than pure water cubes?
extra points for not waterifying my soda :)
 A: If you look at this table you will see that water, even without the phase transition, actually has the highest volumetric heat capacity of any of the substances listed (absent any phase transition materials).
This means that an ice cube is indeed the best you can do for cooling down your drink. Even when you take account of the heat of fusion, water is better than most materials. The only material that comes even close is formic acid with a value around 250 J/g (ice is 334 J/g). Of course your drink will taste HORRIBLE when that dissolves - formic acid is the stuff in some insect bites/plant stings.
If you really want to get as cold as possible, you want to lower the melting point of your water (for example by adding salt) until it's just above the coldest temperature your freezer can achieve. Put that liquid inside a thin-walled container. Put plenty of those cubes in your drink; you will be able to cool the drink all the way to the melting point of your "salt ice".
I found such cubes online (no commercial relationship with this site... just found it with Google).

A: You can try ice cubes made of soda for having only soda. You can try having spherical cubes instead of small cubical ones, which take more time to melt.
Or else try adding some salt to the water and freezing it. It reduces the freezing temperature of ice which makes the ice freeze faster and melt slower.(But 10 gm of salt in 10 litres make a change of 0.1 Celsius change)
You can also try with different shapes of trays which you use, a tray with less surface area, the less surface area exposed, the less it will melt.
Ice made in plastic trays and silicone molds freezes from all sides, trapping impurities and making for a weaker cube. Using an open insulated cooler, which forces a top-down freeze,  making a dense, slow-melting ice.
