Lossy is the best option for most users. The images processed with Lossy algorithms are the smallest optimized images you can get. So, if the speed of your site is paramount and you want the best balance between optimization and picture quality, we recommend you continue to use Lossy optimization.
Glossy is the best choice if you still care about Google Insights but you believe that a slight loss of page speed is an acceptable compromise for a top notch image quality.
Lossless optimized images are pixel-by-pixel identical with the originals, but they offer a smaller size reduction compared to either Lossy or Glossy processed files. If you want your images to remain untouched, then select this option.