Bullock is starring with McCarthy in comedy film 'The Heat', the Contactmusic reported.
"Melissa McCarthy's the hottest co-star of all time! Melissa McCarthy is my hottest co-star," Bullock said.
Though she loved working with McCarthy, it is impossible for Bullock to choose between her other three hot co-stars - Ryan Reynolds, George Clooney and Bradley Cooper.
"I would never compare any of my perfectly hot co-stars, who are equally hot and amazing, to each other," she said.
McCarthy revealed that she and Bullock became firm friends while shooting the film because they both have quirky personalities.
The movie's director Paul Feig said McCarthy and Bullock's great chemistry made their set a lot of fun.
"The great thing is, the girls got along great. They're still really close friends and that chemistry just kind of took over the whole time. It made it very fun. We laughed a lot," he said.
