Let’s go to the quarry and throw stuff down there!
I’d say it was Sneak Previews, since it gave him national exposure at a time when it was difficult to get (mid-70s), at least for a film critic. Ddamn near every television set in the country got PBS back then, so he and Gene Siskel were known not just by people in Chicago, where they lived and worked, but by people in New York City, Miami, Los Angeles, etc. And since they were known all over the country, it was possible for Hollywood PR types to include quoted parts of their reviews in advertising campaigns, since it was a “real” name that people could relate to, if not actually read their reviews.
The whole thing took off from there, with Mr. Ebert’s aforementioned prodigious output helping.