Probably, right now, the best sensor overall image quality (short of medium format) on the market is the 45.7 MP Sony sensor (with a side order of Special Nikon Sauce) found in the Nikon Z7 and D850.
Just behind it is its close relative the 42.4 MP Sony sensor from the A7rII and III.
The difference between them isn't the 3 MP - that's not actually noticeable. It's that the Special Nikon Sauce enables ISO 64, and ISO 64 is utterly noiseless with extended dynamic range, if you have the light for it. At ISO 100, good luck telling them apart (the Nikon version is a slightly newer design, and may be a little bit ahead in one parameter or another, but it's very close).
The 50 MP Canon EOS 5Ds sensor trades a tiny bit of extra resolution for about a stop and a half less low-ISO dynamic range than the class leading Sony/Nikon sensors. In many ways, the Canon 50 MP sensor is actually behind the older Sony 36 MP sensor in the Nikon D800/D800e, Sony A7r and A99, Pentax K1 mk1 and mkII and possibly another camera or two in low-ISO image quality.
In APS-C, the champ is some version of the Sony 24 MP or the newer Sony 26 MP sensor - probably the model with Special Fuji Sauce (although that may be due to the Special Fuji Lenses it's often found with).
Unless you shoot landscape (which I happen to), the right sensor for you may not be the low-ISO image quality champ. You may prefer the fastest sensor around, the sensor with the best video bitrate, the sensor with a particular set of colors you like, or the sensor that comes in the camera that fits your hand, or comes in the camera your favorite lens attaches to. All modern sensors are good enough that any of these are good reasons to choose a system.
. I'm lucky enough that the sensors that match my shooting style happen to come with bodies and lenses I like a lot (my collection includes Nikon Z7 and Fuji APS-C, and what I'm likely to add to right now is my Z system - I can't believe the detail, dynamic range and noiseless performance I'm getting out of that system). I never bought into the D850 (too heavy for how far I hike, especially with the better lenses), nor Sony FF (weather sealing and controls), despite their superb image quality potential, because the Fujis were a better overall fit for me. When the Z7 came out, there was D850 quality in a smaller, lighter, stabilized body with D850 sealing, along with the beginnings of a line of superb smaller lenses (giving up f2.8 )... I have the 24-70, and anxiously await the 14-30 and a lightweight tele zoom (how about a PF diffractive zoom in z-mount, Nikon?).