Looks like I'm choosing Bess Flowers, who has 1,045 acting credits to her name. That's nearly three movies per day which might keep me from going insane in solitary which would be the whole point. And when I got out I could create an epic "Top 1,000 Best Bess Flowers Movies of All Time" thread.
Another good volume pick would be Jane Bartholomew, who appeared in countless movies due to being the "torch lady" in the Columbia Pictures logo for 40 years.
A more recent name would be
Barbara Harris. Probably the hardest working woman in showbiz, she's a "loop group" artist. Her and her troupe of actors provide voice acting for crowd scenes in TV and movies. When they shoot a scene, only the main characters talk, all the background extras mime their performances. Once editing is complete months later, they have 5-12 voice actors come in and do
all the background dialogue, murmuring in a crowd scene, any TV/radio bits one might hear, PA announcers, or generally anything one might hear spoken in the background. Basically all the spoken parts that
aren't the main actors talking.
Easily 2000 credits, whenever she's listed as Loop Group Casting that would count because she always casts herself as one of the voices. Her work includes 40+ films nominated for Best Picture. You'd get her work on Boogie Nights, Pulp Fiction, the Shawshank Redemption, all the Star Trek films from #3 onward, the Jurassic Park movies, most of Clint Eastwood's director credits since 2000, and entire shows like
Stranger Things and
Veep.