The least malleable element in its solid state is generally considered to be bismuth (Bi). Malleability refers to the ability of a material to deform under compressive stress, and in the periodic table, malleability tends to decrease as you move from metals to non-metals and from left to right across periods.
Bismuth is a post-transition metal, and while it does exhibit some malleability, it is much less malleable compared to other metals like gold, silver, or copper. Other elements that are also quite brittle or less malleable include the metalloids and non-metals such as phosphorus or sulfur.
However, considering typical metallic behavior, bismuth is often singled out due to its brittleness compared to more malleable metals.