If you are talking recent history in VAG cars, I think one of the best ever made was the AFN 110bhp engines used pre PD engine around 1998-2001 . They had a great mix of reliability economy and performance.
I really rate the 3.0 TDI engine in my car, the earlier ones used to suffer with injector issues but the later ones seemed to be pretty good with no major problems.
It's hard to tell what the latest engines are going to be like, they are so full of emissions control stuff with DPF's EGR, long life servicing and stripped down inside to reduce weight I wonder about the long term reliability .