What is the best Audi under 20,000 to get?

Anything as long it's used

Any good used audi as long it is maintained properly

Good for you! Life’s too short to drive boring cars like a Camry. 

Obviously RS models are out at that price and they are expensive to run because they are virtually supercars

You might get an S3 around 20k but a newer A3 or A4 would be cheaper to run. TDi s are fast and use very little fuel.( although a bit more than the official figures).

Quattro models are worth considering if you live somewhere with bad winters.

I really don’t understand to constant references about poor reliability of German cars. They are highly regarded in UK and Japanese cars are generally more reliable only because they are simpler.

A used Audi is a poor choice for a first car -- unless you have plenty of money and time for frequent repairs. My one and only Audi had everything electrical fail on it at some point in time during the 3 years I had it. Never again.

None. Audi is a terrible choice unless you are just made of money.

Honda, Toyota, Mazda make much better cars and cheaper to maintain & repair.

repairs are pricey

Audis are driven by morons so the answer is none. 

DO NOT BUY A USED GERMAN CAR. Especially for a first car. You will spend too much for maintenance and probably too much for insurance. The most reliable used vehicles under $20k are Honda Civic, Toyota Corolla, Toyota Yaris Hyundai Elantra, Hyundai Accent, Kia Soul and Kia Forte. All 2018 and most 2019 models should fall under the $20k mark. 

Personally, I would choose the Elantra for comfort and long warranty that would still be in effect if it has under 60,000 miles.  I'm looking at one now on Carvana that is a 2019 model with 15,000 miles on it for $15,990. The warranty will last until at least 2023 or 60,000 miles, whichever comes first. They get 28 mpg City / 37 mpg Hwy.