What is the key regulatory step in cholesterol biosynthesis?

Well, cholesterol bio-synthesis is controlled by the regulation of HMG-CoA Reductase. This happens near the beginning of the pathway, so in my opinion would seem to be the key step in the process.

There are actually 5 steps considered to be the bio-synthesis pathway of cholesterol. They all seem to me as important but it's just my opinion.

Hope it helps.