In my review of the Pentax K-30, I stated what I believe are the four best APS-C sized sensors for digital cameras. I said, "There are four outstanding APS-C sized sensors out there: a 16 megapixel Sony sensor, a 24 megapixel Sony sensor, Fuji's16 megapixel X-Trans sensor, and Sigma's Foveon X3 sensor. Each have there own advantages and disadvantages, but they are each great in their own way."
I was asked why I didn't include Canon's 18 megapixel sensor or Samsung's 20 megapixel sensor in the list of best APS-C sized sensors.
Canon's 18 megapixel APS-C sized sensor, which is found in the Rebel T3i and T4i, is junk. Well, it's actually perfectly fine for capturing images, but compared to the sensors mentioned at the top, it is junk. DxOMark ranks the T4i 96th out of all digital cameras, which is not a good place to be.
Now I'm not sure if the problem is the sensor or Canon's software (or both). But whatever the issue is, the image quality from cameras with this sensor is not nearly as good as from pretty much any other camera with an APS-C sized sensor.
The Samsung 20 megapixel APS-C sized sensor, which is found in the NX200, NX210 NX20, and NX1000, is not a bad sensor. I've done professional work with this sensor. Performance at low ISO is great, although dynamic range and color depth could be just a little better (but they're still not bad). Performance at high ISO is really lacking. Overall, I feel this is a good sensor, but it just doesn't have quite enough upside and a just little too much downside to make the "best" list. But it is a close runner up, in my opinion.