The coldest (the most endothermic) reaction
There is an endothermic nuclear fusion for the elements heavier than iron. On Earth it's most likely a nuclear fission of light elements, like the one occurring in early "dry" thermonuclear bombs, or a fission processes preceding D-T fusion reaction: 

$$\ce{^7_3Li + ^1_0n -&gt; ^3_1H + ^4_2He + ^1_0n - \pu{2.467 MeV}}$$

It's probably not the answer you'd expect, but the absolute values of thermal effects of any chemical reaction would be much lower in comparison.

Special thanks to David Richerby for correcting me!