Cost: 3000 (Receive one free at start) Use: Allows you to build structures.
This is the root of your base, as all buildings will be built from here. Without this you can't build buildings, and without buildings you have no base - hence this is the most important structure you have. It costs 3000 to build a MCV and you must have a service depot - so try to keep the one you start with safe.
The MCV is vital - so taking out an opponents MCV is a major coup. To prevent an enemy rendering you MCV-less it isn't a bad idea to build walls all around your MCV. This will cost you 500 credits but will prevent engineer rushes and result in tanks taking 20 seconds longer to break through the walls to attack your MCV. Protect this building at all costs - if structures were body parts then this would be your heart.