I know cycle legend Miguel Indurain had a resting heart rate of 28 BPM when he was in top shape.
When I was into endurance, running and shit, the lowest I went was 36-37.
Nowadays it fluctuates between mid 40's and high 50's, depending on whether I'm doing cardio.
I recently started doing HIIT again, and have been pushing my rest rate down quite some in a short time.
YIP
Zack