I would say the 6.6.x release is probably the most compatible release with apps and technical addons. You will need 7.0.x if you want to use the new metrics type indexes. Right now I have most of my instances running 7.0.2 and a few heavy forwarders still running 6.6.6 for compatibility of a few technical addons. As far as stability, I usually wait for the .1 release to come out before going to a new version in production. i.e. 6.6.1 or 7.0.1 or higher.
6.6.6 will have the latest patches for the 6.6 release.
Rob
