There are many different arrangements of structures or units that can be used to block off the entrance to a base to prevent a zealot push or zergling rush. The obvious answer is barracks and a supply depot, but I've also seen people barricade their entrance ramps with all supply depots, or with bunkers.

If barracks are being used, the direction of the ramp and other factors makes a difference, since placing the barracks in the wrong place can prevent you from building add-ons later on.

What is the best positioning for an early-game terran ramp barricade, and how does ramp direction (or cliff edge), and information you get from scouting influence it?

To avoid subjectivity, let's say "best" refers to the most common techniques used by experienced players.