There are two (Ok, three if you count Teimani) traditional families of nusach in use - Ashkenaz (the Nusach which people from northern/eastern Europe traditionally prayed in) and Eidot Hamizrach (the Nusach which people from Arab/Muslim countries prayed in).
Then, in the 1700s with the spread of Chassidus, a few new Nuschaos appeared:
- "Nusach HaAri" - the Nusach instituted by R' Shneur Zalman of Liadi (he sifted through [60 IIRC] siddurim to find a Nusach which works according to Halacha, Kabbalah and Dikduk)
- Nusach Sefarad - the standard siddur used by other Chassidim. It combines Nusach Ashkenaz with Eidot Hamizrach (so it would work according to Kabbalah) and is somewhat similar to "Nusach HaAri".
Who edited this Nusach Sefarad? I haven't heard that it be done by "a big Rov". Was it done by different publishers? If so, who says they know anything about Halacha, Kabbalah or Dikduk?