What is your budget?
What sort of fuel economy are you aiming for?
I've got the A6 Avant with the "de-tuned"* S5 supercharged V6 engine that is a pretty good mix of having an engine that you can drive like a torquey diesel when economy is an issue and then drive it like a revvy (7000rpm) V8 if you want.... I think Audi changed the engine around 2015 to a turbo that's less enthusiastic on the revs (and less low-down torque), but more economical / lower emissions.
*they're detuned from the S5's 333bhp to 290bhp/300bhp/310bhp depending if you get one from 2010/2012/2014... the engine tracks the S5 power curve up tot their chosen peak power level (normally around 4k-5k rpm and then they hold that exact power through to ~7000rpm). I think all of them have identical gearbox, clutch, intercooler, etc to the S5, so are remappable to 375bhp-400bhp for £200-£500.... and with a supercharger pulley change, intake (incduction/filters) and exhaust and you should be seeing ~450bhp (probably ~£3k for that though!) and there's some Dutch tuners getting 550bhp (for around €9000!) and they apparently have ~600bhp out of the 3.0 TFSI on their test benches, but I've never seen them advertising those upgrades for sale.
Mines on a mild (375bhp) remap that only cost me an extra £100-odd on my insurance (up from £360pa to £475pa) and I get 20-25mpg with enthusaistic driving and >35mpg on steady cruise.
Note that even the S4/S5 is detuned to stop it giving the RS4/RS5 a bit of a fright (
see Chris Harris's remapped S4 in a drag race with an RS4). Look at the graphs below for the way that the boost pressure gets bled to curb the peak power.... then picture the same for the A6, but just starting even lower down the rev range.
View attachment 94730
Some tuners reckon on an S6-challenging 414bhp just on a remap!!
View attachment 94729