Cooling efficiency of different fliuds?

Hydrogen has one of the highest Specific Heat Capacities for a gas.

I think this table is up-to-date, listing hydrogen with the highest SHC at 2.4, and a higher thermal conductivity, meaning it can exchange heat faster.

If you want to use liquids, check out this one:

However, I believe polluted water will have the same SHC as water in the next patch (rocketry). Polluted water may still be your best bet with high SHC and lower freezing point.

The high SHC of hydrogen is also useful for cooling with wheezeworts, as they cool a gas a certain number of degrees, so hydrogen will get more effective J of cooling per puff.