Can you rewrite or clarify your question? Are you asking about actors whose first role is a Best Picture Winner? Because that idea doesn't directly translate into your question as written. Or are you asking something else? Assuming that this also can be said about your pre1970 version of the suggestion.
And can you please add the title of the Best Picture which each actor has said to acted in in their respective text boxes?
1) As requested by Stephen Atwood, the update is done
2) I realized that if 1917 win the Best Picture at the Oscars, Colin Firth will be the first one on the post-1970 list to have played in 4 best picture winners.
cinephile
To answer your question I will use an example:
Bernard Hill: https://www.imdb.com/name/nm0384060/?ref_=nmls_hd
He played in 3 best picture winner:
1- Gandhi (1982)
2- Titanic (1997)
3- Lord of the Rings: The Return of the King (2003)
You can notice that he played in his first best picture winner in 1982 for Gandhi.
1982 is after 1970 so I put him in the Post-1970 poll.
So the poll is asking which one of those actors (the minimum is 3 characters in Best Picture movies) is the best.
I will make the update that you suggested soon, and I will also say in the description that the minimum is 3 acting credits.