Welcome to the forums, Gianni, I'm sure you will find them very friendly.
The question you are asking is a difficult one. Seven different people are
likely to give you seven different answers
I use Ubuntu 6.06 lts Server, which is kind of cool as it is pure terminal
although you can install a GUI if you wish. It was easy to set up and I
found a great guide called "The perfect server" at howtoforge.com.
Another one that seems to get recommended a lot is CentOS which is a
"de-branded" and recompiled Red Hat Server OS. I have no idea what it is
like.
The simplest answer is to try several and see which one you get on with the
most.
I have just done a quick search at howtoforge and they have guides for both
of these.
Good luck with which ever one you choose.