Due to the surface-enhanced Raman scattering (SERS) effect, SERS labels based on noble-metal nanoparticles loaded with Raman-active molecules are good candidates for ultrasensitive multiplexed assays and in vitro/in vivo imaging. However, understanding how to maximize the brightness of such labels is of paramount importance for their widespread application. The effective differential Raman scattering cross-section (dsR/dO) of SERS labels made of pegylated gold nanoparticles loaded with various Raman active molecules (Raman reporters) is studied. It is found that proper choice of the Raman reporter and of nanoparticle size can enhance the dsR/dO by several orders of magnitude. The experimental results are understood by considering the molecular cross-section for resonant Raman scattering and the local electromagnetic enhancement factor (GSERS) in the nearby of gold nanoparticles. These results are useful to guide the design of SERS labels with improved performances and to provide a reference for the comparison of the absolute value of the dsR/dO of SERS labels based on metal nanoparticles.