Sony Alpha a9 DxOMark score is 92, better than Canon EOS-1D X and Nikon D5.
You can take a look comparison with other cameras:
Sony Alpha a9 Vs. Sony Alph a7R II Vs. Sony Alpha a7II:
Sony Alpha a9 Vs. Canon EOS-1D X Mark II Vs. Nikon D5:
Conclusions from DxOMark:
The Sony a9’s sensor ranks as one of the best we’ve tested at base ISO, with consistently good scores for dynamic range and color depth, and exceptional results for noise. At those crucial mid to high ISO sensitivities for action photographers, image quality is also excellent and on par with flagship Nikon and Canon sports DSLRs.
For the very best image quality, the A7R II offers better results for color, as well as almost twice the sensor resolution, but it isn’t a sports camera and can’t compete for performance against the a9, so they’re intended for different markets.
With many pros already heavily invested in lenses and accessories for those systems, it’s going to be a tricky (not to mention expensive) to switch, but the Sony a9 has both the performance capabilities and image quality that make it a seriously tempting proposition.