Parallel-plate waveguides are among the most common low-loss broadband waveguides in the terahertz frequency regime. One application is microfluidic detection. Adding a groove into one of the waveguide plates leads to a resonant feature of relatively high quality factor (Q-factor), that shifts to different frequencies when the groove is filled with different liquids. We investigate the resonant frequencies and transmission characteristics of different-sized grooves in aluminum plates in order to determine which groove is most suitable for microfluidic sensing. This apparatus is formed by machining grooves of varying geometries into aluminum plates, which are then used to form parallel-plate waveguides. Sub-picosecond terahertz pulses are used to excite the lowest-order transverse-electric (TE1) mode. The output spectrum is analyzed to determine resonant frequency and Q-factor for each groove geometry, then used to determine which groove gives the highest Q-factor, thus increasing the sensitivity of a groove-based microfluidic sensor.