We compare the ability of 11 differential emission measure (DEM) forward-fitting and inversion methods to constrain the properties of active regions and solar flares by simulating synthetic data using the instrumental response functions of the Solar Dynamics Observatory / Atmospheric Imaging Assembly (SDO/AIA) and EUV Variability Experiment (SDO/EVE), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and the Geostationary Operational Environmental Satellite / X-ray Sensor (GOES/XRS). The codes include the single-Gaussian DEM, a bi-Gaussian DEM, a fixed-Gaussian DEM, a linear spline DEM, the spatial-synthesis DEM, the Monte-Carlo Markov Chain DEM, the regularized DEM inversion, the Hinode / X-Ray Telescope (XRT) method, a polynomial spline DEM, an EVE+GOES, and an EVE+RHESSI method. Averaging the results from all 11 DEM methods, we find the following accuracies in the inversion of physical parameters: the EM-weighted temperature T w fit / T w sim = 0.9 ± 0.1 , the peak emission measure EM p fit / EM p sim = 0.6 ± 0.2 , the total emission measure EM t fit / EM t sim = 0.8 ± 0.3 , and the multi-thermal energies E th fit / EM th approx = 1.2 ± 0.4 . We find that the AIA spatial-synthesis, the EVE+GOES, and the EVE+RHESSI method yield the most accurate results.