This article is from the Formula One Motor Racing FAQ, by firstname.lastname@example.org (Mitchell McCann) with numerous contributions by others.
Sounds like a subjective question but almost universal opinion
seems to favour "Grand Prix" starring James Garner, directed by
John Frankenheimer. The plot may not be watchable but the racing
is. Actual race footage is combined with staged scenes which
were filmed during the GP weekends using Lotus F3's dressed up as
F1s. Graham and Phil Hill did some driving and Garner did a lot
of his own driving and was apparently quite fast.