i'd say red hat too, because many of the people on this board use it for such. I don't have a server and i use mandrake but i think mandrake is geared toward desktop use, ipso facto, i would agree with these guys when they say red hat.
Many people on other boards say clackware is the way to go, or FreeBSD, however they are not so mainstream or so user friendly (again i am speaking totally out of hand having had no experience of slack and not any real experience of FreeBSD), so there it is, we're back to red hat.
red hat is also one of the most easy distros to get and one of the ones most people will have already used (so more people to ask if you get in trouble).