Anything 60s or 70s @ Historic Nord-/Südschleife (obviously) Copa Classic B & FL @ any Rallycross track Rallycross in general Formula Vintage Gen1 @ Spielberg Historic Formula Classic Gen1 @ Adelaide 1988 Ginetta G55 GT4 or Supercup @ Kyalami (no idea why I like this combo so much) There also is a long running thread somewhere with combos that people suggested over the years, but I cannot seem to find it.
M1 Procar @ '67 Silverstone SuperV8 @ Watkins Glen Boot. (Nascar layout) Cat Supersport @ Cadwell Park