A6 body stlye. The A6 currently has a campaign to address the many problems (window clips, interior/exterior trim, front bumper attachment problems). My parents have a S4 avant and the only problems so far were the instument cluster and a leaky headlight washer pump.
The only other common problem with the S4 is that the turbo re-circ valves fail. But most people tend to go with aftermarket re-circ valves after awhile. The Bosch design was never "fixed" properly.
Or if you have $85k to burn, go with the RS6. 450hp twin turbo 4.2L V8.