Why is mRNA the least stable?
I assume you mean compared to other RNA like tRNA and rRNA.
RNA is generally unstable because of... chemistry. DNA stands for 2`deoxyribonucleic acid, and the absence of this hydroxy group in DNA is important.
RNA has the normal 2` hydroxy group, and that makes the phosphodiester bond unstable and susceptible to nucleophilic attack and self-hydrolysis.
When RNA is single stranded (like in mRNA), the 2`-hydroxy group can more easily reach the phosphorus atom and cause the chain to be cut. However, when RNA is in the form of a double helix (like in tRNA and lots of rRNA parts), it is limited in movement and the hydroxy group can't as easily reach the phosphorus. For this reason, structure RNA molecules (in a double helix) are much more stable. mRNA is generally unstructured and so is much less stable.