gentoo: As much as I would like to see a Zimbra NE version for gentoo I can not image how do you want to support it. Where do you start the troubleshooting - asking for the USE flags used to build the gcc?
RHES / SLES: there is no need to argue about SLES or RHES its an "AND" because these are the only real Enterprise Linux and it just depends on the geographical location which is number 1 or 2.
debian / ubuntu LTS: I think this are the important distributions for small business that have a Linux environment and need something "like" MS Exchange and don't want to have a MS server.
(just my experience from work as support agent for one of biggest server vendors)