In the past I have shot with a nearly full kit of Hasselblad FE lenses, a Rollei 6008/Hy6 system with mostly Schneider lenses, a Mamiya 7II, Plaubel Makina, and Contax 645. In 35mm I have gone with Leica M and R, Zeiss ZF and ZM, Contax G and many more. In large format I have used Schneider Apo Symmar L's and Super Symmar XL's...
As a whole, the Leica S lenses are not just better, they are in a different league entirely. There are specific lenses in other systems that are spectacular, but they always seem to be a step behind in some measure than the S lenses. Either they are slower, or they are not as good wide open, or they have more fringing or distortion etc. For example, I shot with the 250mm Zeiss Superachromat on the S2 with an adapter. It is a spectacular lens. But the performance at f/5.6 was similar to that of the S 120mm at f/2.8, only it had a lot more vignetting. The S lenses are a tour de force, but of course they are extremely expensive and only work on the S bodies so a lot of people have not had a chance to really get a handle on how good they are. As I have said, I have shot with a lot of the world's best lenses, and the S lenses are so clearly better than pretty much anything else that it is not really worth arguing about. For example, I would be happy to make a 1x1.5 meter print from the 120mm shot wide open at 2.5...or the 35mm shot at 2.5. or the 45mm at 2.8 etc etc. Of course I would not go out and intend to shoot wide open for every shot, but that is the feeling with the S lenses...you don't have to worry about the lens at all...every aperture, every focal length, every distance...they just give you clean, sharp, low-distortion, fringe free, high contrast images with consistent color and look throughout the whole range. And no, I am not being paid by Leica, and sure you can say this is confirmation bias because I own an S system, but I also own a hell of a lot of other systems that I still love using, (Mamiya 7II and Konica Hexar for example), but when it comes to technical prowess, I would put the S lenses up against anything available on the consumer market.
Besides the S lenses, the best lenses I have used are the Mamiya 7 lenses and the Schneider lenses for the Hy6, so I have no doubt that the Schneider lenses for Phase are also very very good. I am also very very skeptical that they are better than the Leicas. I would be happy to see a comparison though.