You just want DR? No other factors?
Then Sony's rx1 is best mirrorless with highest DR, probably better than the 5d mkiii as well.
Yes, only DR. I got the rest covered with my 5D3.
Improved DR at base ISO is the one important thing Canon has failed to deliver since 2008.
The RX1 is great, but it has a fixed lens that is too long for my work.
Since the RX1's lens is too long for your work, then no, you're not just interested in DR - you need a system with good wide lenses (unless you plan to attach your Canon lenses, which rather defeats the point of getting a small camera). The best there is M43, where the wide end includes the much-lauded Panasonic 7-14mm, Olympus 9-18 mm, Olympus 12mm and Panasonic 14mm (all x2 for FF equiv.)
The OM-D's Sony sensor has very good dynamic range; I've not compared it directly with my 5D or 5DIII, but I can say that it doesn't seem to have the 5DIII's banding problem (but then neither does the 6D). That said, Olympus is supposedly announcing an even higher end OM-D in September; perhaps its sensor will be even better than the current OM-D's (the EP5's may be too, for all I know). Olympus and Sony are collaborating in various ways - Sony is getting Olympus's remarkably IBIS system, and in exchange Olympus is getting improved Sony sensors, so there may be further improvements before too long.
Of course, if you're very patient, the results of this development - assuming they ever show up - will make even the D800 look obsolete:
With luck such a sensor will end up in Panasonic M43 bodies....