It depends whether you mean in the UK or the US. For example, Tom Jones, which originally had a UK X certificate, won Best Picture in 1963. But in the US, Midnight Cowboy was the first X certificate Best Picture winner.

It's also worth adding that X in America was often considered synonymous with pornography, unlike the UK, where it simply meant a film for grown-ups. Of course, X in America was later replaced with NC-17 in the early 1990s (the first NC-17 rating was Henry and June, from memory), but that rating is still considered commercial suicide. Americans really need to get over their prudery, to my mind.

By contrast, in the UK, we replaced the X rating in 1982 with 18 (and at the same time, replaced the confusing A and AA with PG and 15).