Mya Mason in Gangbang Squad 7.
http://xhamster.com/movies/1555278/mya_mason_gbs7.html
Her pussy seems very tight on the initial penetration at 4:00 or so, but the real action begins at 12:50 where she's on her back and the guys take turns pounding her. One guy nearly gets squeezed out by her tight pussy, and makes her scream when he goes balls deep. Eventually his place is taken by Shane Diesel, who really makes her scream at around 15:10. She uses the phrase "oh my God" a lot in throughout the scene. The anal and dp sequences seem to be heavily edited, so a lot of her screaming in the second half of the scene is cut out, but it's fairly obvious she's struggling here, too.
Another one to look for is Avril Hall in Big Dick for a Cutie 5. She spends most of the scene gritting her teeth and wincing as she tries to handle the guy fucking her. No wonder she's smiling when she gets the cumshot.