In the last decade, aluminum nitride (AlN) has proven to be an attractive material for both linear and nonlinear integrated photonics, due to its large transparency window, its low propagation losses and the presence of both second- and third-order nonlinear optical susceptibilities. However, the investigation and optimization of AlN layers for photonic applications have been scarcely explored in the literature so far, although it represents an important stepping stone upon which will impact the final performance of AlN-based devices. Here we present a systematic comparison of various types of AlN epilayers grown on sapphire by metalorganic vapor-phase epitaxy. Optical losses are measured from waveguides and microring resonators fabricated from different AlN epilayers. The results are analyzed through a comprehensive material characterization study, showing the need to optimize growth techniques to further push the performance of AlN-on-sapphire photonic devices.