The 27-year-old Scot has only been beaten twice in Cardiff's last six league games and has been on fine form for the table-toppers.
"I know I'm biased, but for me David is the best goalkeeper in the division," Mackay told Wales Online.
"He has a quiet authority about him. He does not speak a lot, but when he does speak everyone listens including the coaches and myself.
"He's got such a calm demeanour about him and that is a huge part of it. Calmness in a goalkeeper is vital. David has an authority over the back four too and he instils confidence in them.
"I think I can count on the fingers of one hand the times when David has held his hand up and said 'sorry guys I was at fault'.
"That is massive testament to him. He's a top, top keeper."
Mackay also believes that if Marshall continues to perform at his current level then he will soon be adding to his five Scotland caps.
"He just needs to keep doing what he is doing in the Championship week in and week out now," he said. "Then if he can get into the Scotland squad again the emphasis is on him then to go there and show (manager) Gordon Strachan just what he is made of."