This paper presents a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition metal dichalcogenides (TMDs) and oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin-orbit coupling are calculated using the G0W0 approximation and compared with different density functional theory (DFT) descriptions. The study discusses pitfalls in GW calculations for 2D materials and possible solutions. The monolayer band edge positions relative to vacuum are used to estimate band alignment at heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. The q-dependent dielectric functions and effective electron/hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to estimate exciton binding energies. The paper focuses on trends and correlations in the electronic structure rather than detailed analysis of specific materials. All computed data is available in an open database. The study investigates the stability of 216 monolayer TMDs and TMOs, finding 171 to be stable (defined by a negative heat of formation relative to standard states). These results extend the LDA-based stability analysis of previous work. Out of the 171 stable monolayers, 51 are non-magnetic and nonmetallic, and their band structures are calculated using the G0W0 approximation with spin-orbit coupling included. The convergence of the G0W0 quasi-particle energies is discussed in detail. The G0W0 band gaps and band edge positions are compared to Kohn-Sham DFT using different exchange-correlation functionals. The band gap is generally well reproduced by the GLLB-SC functional, while the LDA provides a surprisingly good description of the band gap center. An empirical formula for estimating the band edge positions from the electronegativities of the constituent atoms is found to deviate significantly from the first-principles results due to charge transfer from the metal to the oxygen/chalcogen atoms. The static q-dependent dielectric function of all compounds is calculated and discussed. The effective charge carrier masses are derived from the G0W0 band structures and used as input to an effective 2D model for the exciton binding energies. The results reveal a large degree of variation in the electronic properties of the investigated materials. For example, the materials MX2 (X=S, Se, Te and M=Cr, Mo, W) have direct QP band gaps in the range 0.9–2.5 eV, while all other compounds have indirect gaps in the range 0.5–7.0 eV. The band gap centers (relative to vacuum) span from -8 eV for some of the oxides to above -5This paper presents a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition metal dichalcogenides (TMDs) and oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin-orbit coupling are calculated using the G0W0 approximation and compared with different density functional theory (DFT) descriptions. The study discusses pitfalls in GW calculations for 2D materials and possible solutions. The monolayer band edge positions relative to vacuum are used to estimate band alignment at heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. The q-dependent dielectric functions and effective electron/hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to estimate exciton binding energies. The paper focuses on trends and correlations in the electronic structure rather than detailed analysis of specific materials. All computed data is available in an open database. The study investigates the stability of 216 monolayer TMDs and TMOs, finding 171 to be stable (defined by a negative heat of formation relative to standard states). These results extend the LDA-based stability analysis of previous work. Out of the 171 stable monolayers, 51 are non-magnetic and nonmetallic, and their band structures are calculated using the G0W0 approximation with spin-orbit coupling included. The convergence of the G0W0 quasi-particle energies is discussed in detail. The G0W0 band gaps and band edge positions are compared to Kohn-Sham DFT using different exchange-correlation functionals. The band gap is generally well reproduced by the GLLB-SC functional, while the LDA provides a surprisingly good description of the band gap center. An empirical formula for estimating the band edge positions from the electronegativities of the constituent atoms is found to deviate significantly from the first-principles results due to charge transfer from the metal to the oxygen/chalcogen atoms. The static q-dependent dielectric function of all compounds is calculated and discussed. The effective charge carrier masses are derived from the G0W0 band structures and used as input to an effective 2D model for the exciton binding energies. The results reveal a large degree of variation in the electronic properties of the investigated materials. For example, the materials MX2 (X=S, Se, Te and M=Cr, Mo, W) have direct QP band gaps in the range 0.9–2.5 eV, while all other compounds have indirect gaps in the range 0.5–7.0 eV. The band gap centers (relative to vacuum) span from -8 eV for some of the oxides to above -5