I'm not far into Unity, but it seems quite decent. Also no bugs or serious framerate drops encountered thus far. But I'm holding my judgement on this one until I play it a bit more.
Haven't played Syndicate. Other than that:
Tier 1
AC II
AC Brotherhood
AC IV
Tier 2
AC (for its time)
AC III
AC Rogue
Tier 3
AC Revelations
AC III Liberation