This document presents a comprehensive first-principles study of the electronic structure of 51 monolayer transition metal dichalcogenides 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 authors discuss the convergence issues of GW calculations for 2D materials and propose solutions. The monolayer band edge positions relative to vacuum are used to estimate band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. Additionally, 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 computed data, including relaxed structures, DFT and G0W0 band structures, absolute band edge positions, effective masses, and dielectric functions, are available in an open database. The study reveals significant variation in the electronic properties of the investigated materials, with direct QP band gaps ranging from 0.9 to 2.5 eV and indirect gaps in the range of 0.5 to 7.0 eV. The effective masses vary by almost two orders of magnitude, and the q-dependent dielectric functions show strong $q$-dependence. The results highlight the importance of considering both the band gap and absolute band edge positions, effective masses, and dielectric functions for predicting the usefulness of 2D materials in various applications.This document presents a comprehensive first-principles study of the electronic structure of 51 monolayer transition metal dichalcogenides 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 authors discuss the convergence issues of GW calculations for 2D materials and propose solutions. The monolayer band edge positions relative to vacuum are used to estimate band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. Additionally, 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 computed data, including relaxed structures, DFT and G0W0 band structures, absolute band edge positions, effective masses, and dielectric functions, are available in an open database. The study reveals significant variation in the electronic properties of the investigated materials, with direct QP band gaps ranging from 0.9 to 2.5 eV and indirect gaps in the range of 0.5 to 7.0 eV. The effective masses vary by almost two orders of magnitude, and the q-dependent dielectric functions show strong $q$-dependence. The results highlight the importance of considering both the band gap and absolute band edge positions, effective masses, and dielectric functions for predicting the usefulness of 2D materials in various applications.