Silverstone has very good quality as it is...
I would like to have Circuit Catalunya - Montmeló, that is also known as Barcelona.
Why? Because it is used for testing purposes and races across huge amount of different categories, from Formula 1 to Blancpain gt, it is even used for prologue from WEC, and I believe rfactor 2 have stated that they are heading some kind of direction towards WEC. It is a must in almost any sim to have this circuit in decent quality.
I personally enjoy driving in sims just a few circuits, Barcelona (Circuit Catalunya - Montmeló) and Nurburgring (Combined, Nordschleife, Nurburgring GP) and maybe Spa sometimes.
Right now I have 3 different versions in rfactor 2 (imagine that if I have 3, maybe it is an indicator that none of them satisfies me completely): Racerfactor Barcelona 2013 1.88, AC-Barcelona 1.06 (from Assetto Corsa I guess) and Sim Racing Hub F1 2019 - Circuit Barcelona Catalunya. The quality of all three is decent, but not up to Nordschleife from S397 for example.