Your comment seems to say the 1DsMkIII had no low ISO noise/banding. As someone considering purchasing one I wanted to confirm that positive endorsement (if that's what it was!)
The 5DMkIII and 6D are a confusing pair of cameras. Like the earlier poster I'd like to mix the features differently, some from each... my 5DMkIII a poor copy, I believe, but wished for more improvement in IQ over the 5DMkII.
Well they all have read noise at low ISO, all a lot more than the exmor stuff, but the 1Ds3 did have among the least low ISO banding that Canon had ever released and some of the better ISO100 read noise for a Canon.
From what I hear and have seen and in some, but not all, cases tested, it seems like for the FF cams from Canon that 1Ds3,1DX,6D all have noticeably less banding than the 5D3/5D2 (5D3 completely cures it in one direction but since it still has it so strongly in the other direction it doesn't really do much anything to help). From what I recall 1Ds3 has no horiz banding and a little bit of vertical and the 5D2 has a lot of both and the 5D3 has no almost vertical but a lot of horiz. Those three (1Ds3/6D/1DX) also all have a bit less read noise than the 5D3/5D2 as well at ISO100 (5D3 is actually the worst of them all for ISO100 read noise, although the degree that it is worse than the 5D2 in that regard is so minor that I don't it is possible to notice real world). Some say the 6D has the least banding of these Canon FF cams, I didn't check that out carefully myself yet.
1DX/6D and even 5D3 (and even 5D2) all have better high ISO DR than the 1Ds3 though (even starting by just ISO800 or even 400 really, not even talking high). And at say ISO3200 the 1Ds3 DR actually does fall quite far behind the 5D3 and bit farther still than the 6D and even a touch yet than the 1DX. Even compared to the 5D2 it doesn't even fair so well at ISO3200.
1DX/6D/5D3 all have better SNR than the 1Ds3 across the range (although SNR is so good that at ISO100 it doesn't matter that much and with the better color filters and less banding and a bit less read noise I'd say 1Ds3 does ISO100 better than the 5D3 even if the SNR is worse outside of the darkest tones).
1Ds3 appears to have the least color-blind color array filter when shooting under outdoor lighting of all those cameras. 6D may be the most color blind. In what ways and what the difference means exactly is very complex and hard to say.