IMHO, 9.1 ... I greatly appreciate SUJ. OTOH you may want to remove debugging stuff from the kernel, and maybe the sound devices... Also check the freebsd.org Release Notes for changes (v6, v7, v8, v9... even).
Sometimes, though, one can get by with CURRENT. (Be aware that pkgng is the default now AFAIK, maybe unless opt-out..).
.........
Others though know more than I do about it.