For classic F1 I still use good old rFactor, for which I have several F1 mods installed: 1937, 1965, '67, '71, '75, '79, '90 and 1991. There are several more, but some are just better than others.
I also have a nice Formula 5000 1972 mod installed, and somewhere on disk I have two Formula 2 mods too. (1976 and 1978 IIRC)
And of course there's still Project C.a.r.s. (Not accepting new buyers at the moment though) that has five modern monoposto's and the Lotus 49, 79 and 99T.