Guetzli is great. Here's a better comparison (same filesize) done by one of the authors [1].

I'm working on a more practical tool [2] which produces comparable results [3].

[1] https://drive.google.com/drive/u/1/folders/0B0w_eoSgaBLXQk1V....

[2] http://getoptimage.com

[3] http://i.imgur.com/qmIwJGw.jpg