Since you didn't ask specifically for Linux distro (although it may be implied), I would point towards OpenBSD as well. If you are interested in security, it might be an intersting distro to play around for building firewalls¹ and the like.

¹I have two running at home on minimal hardware. One SMB / Web Proxy / internal DNS / jack of all trades server and one external facing firewall. Both have been very reliable.