Any illumos distro will be better at ZFS than anything Linux-based. Granted, the amount will vary by distro; last I saw, Ubuntu had relatively good support. Beyond that, I'm personally quite fond of the overall design and polish of openindiana, at least. One example is how it automatically handles ZFS snapshots and boot environments with the package manager - all the pieces are there to do that on other systems, but setting up the integration is harder.