The peppy 1.8T is a better choice than the 2.0-litre petrol which is a little weak and sluggish. You won't be disappointed by any of the other engines in the line-up though - the 2.7T with 230bhp is a particularly good unit and provides strong performance while the 2.4-litre, 2.8-litre and 3.0-litre engines are all smooth with plenty of poke. In terms of diesel powerplants, the 1.9 TDI is the choice for economy and copes well in everyday situations but if you regularly tow or carry heavy loads then the 2.5 TDI (it was available in 155bhp, 163bhp and 180bhp during it's lifespan) is the better choice.
If it's performance you're after then seek out an S6 or RS6 model - both came with quattro four-wheel drive as standard and the latter covers the 0-62mph benchmark in just 4.5 seconds.
The 180bhp 2.5 TDI and 2.4-litre petrol models and above are available with the quattro four-wheel drive system which offers superb traction, particularly out of corners. It's standard on the S6 and RS6 models and due to their high power outputs they certainly need it - especially in the wet. The rest of the range is better suited to cruising the motorway than blasting along back roads thanks to plush suspension, well weighted steering and easy controls.
Even when fully loaded the A6 Avant doesn't feel cumbersome.