What substance has the lowest solubility product?
The $K_\mathrm{sp}(\ce{Co2S3})$ value of magnitude of $10^{-124}$ appears in paper by Goates et al. [1].
The refined &quot;thermodynamic&quot; value of $\pu{2.6E-124}$ that you've listed and is used by numerous textbook up to these days has been proposed by Waggoner [2].
However, thirty years later (late 1980s) there's been another study by Licht [3], which showed significant deviation from the previous studies due to two factors:

A new value of the free energy of sulfide ion formation for aqueous solutions  has been used:
$$Δ_\mathrm{f}G^\circ(\ce{S^2-(aq)}) = \pu{(111 ± 2) kJ mol-1}$$
Previous value [1] was
$$Δ_\mathrm{f}G^\circ(\ce{S^2-(aq)}) = \pu{20.64 kcal mol-1} \approx \pu{83.68 kJ mol-1}$$

Free sulfide activity $a(\ce{S^2-})$ used for $K_\mathrm{sp}$ determination

$$
\begin{align}
\ce{M_xS_y &amp;&lt;=&gt; x M^{$2y/x$+} + y S^2-} &amp;\qquad K_\mathrm{sp} &amp;= a(\ce{M^{$2y/x$+}})^x \cdot a(\ce{S^2-})^y \\
\ce{HS- &amp;&lt;=&gt; H+ + S^2-} &amp;\qquad K_2 &amp;= \frac{a(\ce{H+})\cdot a(\ce{S^2-})}{a(\ce{HS-})}
\end{align}
$$

has also been misinterpreted in early studies by acidification of hydroxyl $\ce{OH-}$ instead, erroneously substituting $K_2$ by $K_\mathrm{w}.$


Table I from [3] lists more recent $\mathrm{p}K_\mathrm{sp}$ values, among which the one for $\ce{Co2S3}$ that has increased significantly $(\mathrm{p}K_\mathrm{sp}(\ce{Co2S3}) = 49.9,$ $K_\mathrm{sp}(\ce{Co2S3}) \approx \pu{1.26E-50}).$
Five least soluble sulfides from that table are:
$$
\begin{array}{lrc}
\hline
\ce{M_xS_y} &amp; \mathrm{p}K_\mathrm{sp} &amp; K_\mathrm{sp} \\
\hline
\ce{Ir2S3} &amp; 196.3 &amp; \pu{5.0E-197} \\
\ce{Bi2S3} &amp; 115.1 &amp; \pu{7.9E-116} \\
\ce{Mo2S3} &amp; 107.8 &amp; \pu{1.6E-108} \\
\ce{Ni3S4} &amp; 104.5 &amp; \pu{3.2E-105} \\
\ce{In2S3} &amp; 96.3 &amp; \pu{5.0E-97} \\
\hline
\end{array}
$$
So that it seems like the new champion (at least among sulfides) is iridium(III) sulfide $\ce{Ir2S3}$ with $K_\mathrm{sp} = \pu{5.0E-197}$.
References

Goates, J. R.; Gordon, M. B.; Faux, N. D. Calculated Values for the Solubility Product Constants of the Metallic Sulfides. Journal of the American Chemical Society 1952, 74 (3), 835–836. DOI: 10.1021/ja01123a510.
Waggoner, W. H. Textbook Errors: Guest Column. The Solubility Product Constants of the Metallic Sulfides. Journal of Chemical Education 1958, 35 (7), 339. DOI: 10.1021/ed035p339.
Licht, S. Aqueous Solubilities, Solubility Products and Standard Oxidation-Reduction Potentials of the Metal Sulfides. Journal of The Electrochemical Society 1988, 135 (12), 2971. DOI: 10.1149/1.2095471.