Fano resonance is a sharp and asymmetric spectral feature that can be used for refractive index sensing. In this paper, we propose a Fano resonance sensor based on the coupling between a nanoring resonator and a metal-insulator-metal (MIM) waveguide. The nanoring resonator is fabricated in the middle of the MIM waveguide, and the two structures are coupled with high-field confinement. The transmission spectrum of the coupled structure shows a Fano resonance, which is sensitive to the refractive index of the surrounding medium. The sensitivity of the sensor is estimated to be 1700 nm/RIU, which is comparable to the sensitivities of other Fano resonance sensors. In addition, the designed sensor achieves the first-ever FOM and Q factor values of 4300.25 RIU-1 and 4310, respectively, for plasmonic MIM sensors. The proposed sensor is simple to fabricate and can be used for a wide range of refractive index sensing applications.