as mentioned you can not go wrong with stable distros.
RHEL, or its fork CentOS
in that order as they are all very stable, have a long support life, with RHEL/CentOS being the longest, that I am aware of, 5yr is standard support for the life cycle of RHEL, 7yr with extended and that can be pushed to 10yr, but ONLY via RHEL, not any of their forks.
Debian, IIRC has an 18-36mo life cycle, sadly i have no clue what the life cycle of slackware it. it seems to go back a long ways, but there is some information out there that a fair amount of the older release were dropped from support.
This is why for any long term support server i go with either CentOS and for a business (larger then 50 users or so) ill spend the bucks and invest in a RHEL extended support contract for 7yrs.