In search of the optimal combination of basis set and exchange-correlation potential, we have investigated the dependence of the atomization energies (D-0) and reaction enthalpies (Delta H) for a set of 44 molecules using gradient-corrected density functional theory. Of the Six functionals tested, those that include a portion of the exact (Hartree-Fock) exchange perform best and yield D-0 values that generally lie within 3-5 kcal/mol of the experimental value. For the functionals in which pure DFT exchange is employed, the errors in D-0 are instead on the order of 8-10 kcal/mol. Conversely, reaction enthalpies show slightly better agreement with experimental results when pure DFT exchange is employed. For both D-0 and Delta H the four Gaussian basis sets 6-31G(d,p), 6-311G(d,p), cc-pVDZ, and cc-pVTZ show similar behavior. The most accurate predictions were obtained using the largest cc-pVTZ basis. There is a significant variation in these energies obtained with the various functionals depending on the basis set employed.