I've had the 1D mark II, IIn, 1DS, 1DS Mark II, and 5D bodies.
The ONLY body that betters the 5D in image quality is the 1DS Mark III.
The 5D provides better IQ then the Mark I and Mark II bodies of DS series, both of which were too noisy at ISO 800. Only body build and ergonomics of those better the 5D, but these attributes do nothing for IQ.
I really do not understand why anyone would buy a used 1DS, or 1DS Mark II....great bodies sure, and straight out of camera jpgs very good as well, but that is it.
I think a better body at the expense of image quality, especially if a lessor costing body provides higher IQ shows priorities I personally do not understand.
To be fair, I'm not especially happy with the ergo's of the 5D, nor it's lack of sealing, however the prime directive of a DSLR is IQ, and everything else is 2nd.