The GDI Harvester
The harvester is usually the first built unit on a new game. It gets you quest items as well as much needed resources. Eventually, most smart players get rid of them. If you can be stable and play in one alliance for ever then you don't need them. On the other hand, if you are like me and enjoy a challenge, keeping a few might not be a bad idea. Eventually when you have maxed out on bases you can get, you can switch to them for continuance.