When we go to Hong Kong, we will usually eat at least once at Tim Ho Wan. So far we have tried the outlets at Sham Shui Po and Central.
I find the Sham Shui Po outlet more authentic, though it is somewhat more inconvenient to get to (again it depends where you are staying).
Here are two videos of our experience at the Sham Shui Po outlet.
Tim Ho Wan at Sham Shui Po for dinner
Tim Ho Wan at Sham Shui Po for brunch
My favourite would be the char siew buns. Always have them.