That would be CentOS
If anyone suggested Ubuntu as being the most lightweight then they have no idea what they are talking about, Ubuntu is the most user-friendly distribution and it's designed to make it easy for windows users to switch to linux but it's definitely not what you want to run on a server (I have only seen people using Ubuntu on their laptops)
EDIT: the minimalist versions of CentOS are not user-friendly at all and it's not worth bothering with it, the differences are too small to notice, it's best to go with whatever you feel comfortable with