Generally, the newer the version, the better. However, it should be kept in mind that some tools (such as g_mmpbsa) are only compatible with older versions.
I am doing most of the work with Gromacs 2016.6 and only Gromacs 2021 to perform gmx mdrun.
Please be aware that some topologies obtained with Gromacs 2021 are incomplete, as myself and other researcher found dihedral angles missing in the topol.top file (probably this will be fixed in subsequent versions).