The Horton-Strahler number, also known as the register function, provides a tool for quantifying the branching complexity of a rooted tree. We consider the Horton-Strahler number of critical Galton-Watson trees conditioned to have size $n$ and whose offspring distribution is in the domain of attraction of an $\alpha$-stable law with $\alpha\in [1, 2]$. We give tail estimates and when $\alpha\neq 1$, we prove that it grows as $\frac{1}{\alpha}\log_{\alpha/(\alpha-1)} n$ in probability. This extends the result in Brandenberger, Devroye & Reddad [6] dealing with the finite variance case for which $\alpha=2$. We also characterize the cases where $\alpha=1$, namely the spectrally positive Cauchy regime, which exhibits more complex behaviors.