In January 2009, I bought my Canon 5D Mark II. Iit was a state of the art full frame camera with what Canon said was the best image quality of their lineup at the time.
One year later, I bought the Leica M9 for 2.5x the cost of the 5D Mark II, and it absolutely blew the doors off it. I was constantly amazed at how rich, sharp and 'deep' the images of the M9 looked in comparison with the 5D.
Having said that, the lenses were also an important factor. The main lenses I used with the Canon were the 24-105L and 35L, neither of which are known for their sharpness. I bought the 70-200 f4L and 16-35L later, and while they are very decent lenses, they still didn't have the oomph of the Zeiss 21mm Biogon or 75mm Summarit.
In many ways, I prefer the images of my tiny m43 cameras like the Panasonic GH3, GH4, GX85 and GM1 to the 5D Mark II. The Panasonic f2.8 zooms and Olympus f1.8 primes are extremely usable, especially the Olympus primes. Their image quality isn't as good as the M9, but they are far smaller, lighter, more flexible and less expensive to replace.
These days, I'm sure that a Nikon Z6 or Sony A7 III has better image quality than the M9, and a 2.5x the cost, but the are still more bulky in terms of lenses and overall shape. Not to mention the upcoming Panasonic S1, which promises to be slightly bigger and heavier than my Canon 5D Mark II from 2009.