This paper presents a comprehensive comparison of online loop gain monitors for microgrid converters in terms of steady-state, dynamic and anti-noise performance as well as implementation etc. In the light of the perturbating signals, two categories of monitors can be identified: wide-band monitors and single-frequency monitors. Pros and cons of each loop gain monitor, which were not clear before, are now systematically summarized and compared. The measured loop gain by different monitors are essential to performance characterization and stability study for various power systems that involve such converters, including ac, dc, and hybrid ac-dc power generation, transmission and distribution systems.