In choosing a polarizer, look for two key things:
Uniformity of transmission — color neutrality. A spectral transmission graph shows this by how flat/straight the plot is.
Percent transmission — how bright the polarizer is. Greater transmission means a higher shutter speed.
Polarization efficiency — how pronounced the polarization effect is. For high altitude, I’d like to have a much less efficient polarizer because polarization increases about 3% for every 1000 feet of elevation. So at 10000' elevation, there is 30% greater polarization effect which is why I usually avoid polarizers at altitude—a dark band can ruin an image.
Shown below are charts from Breakthrough Photography* showing spectral transmission. More information at https://breakthrough.photography. Polarization efficiency is not shown in these charts, but it’s ample (more than I want) at high altitude.
The Breakthrough Photography X4 CPO shows exceptionally neutral color transmission. The Hoya CPL and Singh Ray CPL shows a bias to green/yellow/red (less blue).
* I have not independently verified these charts.