  Ab initio many-body perturbation theory within the $GW$ approximation is a
Green's function formalism widely used in the calculation of quasiparticle
excitation energies of solids. In what has become an increasingly standard
approach, Kohn-Sham eigenenergies, generated from a DFT calculation with a
strategically-chosen exchange correlation functional ``starting point'', are
used to construct $G$ and $W$, and then perturbatively corrected by the
resultant $GW$ self-energy. In practice, there are several ways to construct
the $GW$ self-energy, and these can lead to variations in predicted
quasiparticle energies. For example, for ZnO and TiO$_2$, reported $GW$
fundamental gaps can vary by more than 1 eV. In this work, we address the
convergence and key approximations in contemporary $G_0W_0$ calculations,
including frequency-integration schemes and the treatment of the Coulomb
divergence in the exact-exchange term. We study several systems,and compare
three different $GW$ codes: BerkeleyGW, Abinit and Yambo. We demonstrate, for
the first time, that the same quasiparticle energies for systems in the
condensed phase can be obtained with different codes, and we provide a
comprehensive assessment of implementations of the $GW$ approximation.
