  The differential Galois group of an $n^\mathrm{th}$ order linear differential
equation is the symmetry group of its solutions; it is an algebraic subgroup of
$\mathrm{GL}_n(\mathbb{C})$. More generally, if $G$ is a simple complex
algebraic group, the differential Galois group of a $G$-connection is an
algebraic subgroup of $G$. A connected reductive subgroup of $G$ is said to
have maximal degree if it has a fundamental degree equal to the Coxeter number
of $G$. We give a complete classification of these subgroups and generalise a
theorem of Katz on linear differential equations by giving a criterion for the
differential Galois group of a $G$-connection to be reductive of maximal
degree. As an application, we determine the differential Galois groups of
certain $G$-connections that play an important role in recent work on the
geometric Langlands program: connections on $\mathbb{G}_m$ with an (irregular)
"Coxeter" singularity and possibly an additional regular singular point.
