The DI V6 in the A6 has much less issues than the non-DI does. The pre-facelift models in particular have too many electrical niggles and expensive maintenance for me to recommend one.
The E350's motor (gas) is tried and true. However, they can suffer from small failures (pano sunroof, keyless go, command, etc.) that end up being $$$ at the end of the day- especially if you have the 7-speed's temperamental valve body.
The E60 I6, much like a Jag S-type with the V6, will suffer from leaky valve cover gaskets. Both can come with the 6hp26 transmission, which can suffer from failure of the "mechatronics" unit if has an undetected leak. Nav failures don't happen with the E60 if you get the updated system in the 2010. 528i is relatively bullet proof but slow. 535i's have been sorted out by now and have extended warranties on the turbos, courtesy of BMW. 550i's I know less about.
For me, based on available features (where the Jag really falls behind), with reliability taking a precedence:
1. E350/528i (The 528i I found was too good to pass up)
2. 535i/550i (low-option)
4. 535i/550i (with dynamic drive all that)