All three are solid choices. Best thing to do is install all of the linux's you're interested in on a virtualbox image and see which one you prefer.

Personally I use either Debian or Ubuntu on all of my servers with the webmin interface. If you're looking to squeeze every once of performance out of the image then I would suggest FreeBSD or slackware (if possible) but the learning curve is much higher wit those two.

In the grand scheme of things they're all the same with minor differences and any linux can be customized to exactly what you need.