We demonstrate a refractive index (RI) detection technique based on an array of nanometer scale slot-antennas milled in a thin gold layer using a single lithographic step. Our experimental Figures of merit (FOMs) of 140-210 in the telecom wavelength range approach the fundamental limit for standard propagating SPR sensors (∼250). The underlying mechanism enabling such high FOMs is the combination of a narrowband resonance of the slot-antennas with degeneracy breaking of Woods anomaly under slightly non-perpendicular illumination. In addition, we study the sensitivity of the thickness of the analyte layer. This concept can be easily tuned to any desired wavelength and RI range by modifying the slot dimensions and the array spacing, thus rendering it highly attractive for numerous sensing applications.