technically a stage 3 B5 S4 is the best bang for buck as you can pick up 500hp with 6spd for ~10k. But the sedans are ugly. And as I have found out the exhaust for stage 3 level powers is on the ricey side of the spectrum.
The RS6 is good value when it isn't in the shop. A 6spd car with aftermarket suspension will eliminate 80% of the headaches and majorly ups the bang for buck. My car had the suspension done but the trans was a ticking time bomb according to DHall's vin analysis. IMO ttboosts car is the best value of all the RS6's on the market unless you have the knowledge, facilities, & time for the swap yourself AND just really want to do it yourself.