This thread is full of inaccurate and false statements. The Anshei Knesses Hagedolah were mesaken our tefillah more than 2000 years ago. We have no idea what the exact words of their nusach were, since Chazal actually forbade tefillos to be written down. (See Shabbos 115b “????? ????? ?????? ????”). They were only written later (I believe first by R’ Amram Gaon) because of ?? ????? ??’.
In any case, it seems that it doesn’t halachically matter so much. As long as one says the parts of tefillah that Chazal instituted and doesn’t say anything objectionable or false, he is yotzei tefillah. For example, the fourth bracha of Shmoneh Esreh is a request for knowledge. Therefore, it doesn’t matter whether one says ???? ???? ??? ???? ????? or ???? ???? ????. Neither would be considered ???? ????? ????? ?????.
If Chazal had established a specific nusach for tefilla that we were halachically bound to follow, mishnayos such as ????? ?? ?? ????? ???? ?????, ??? ??? ????? ???,??????????, ?????? ???? (Brachos 5:3). Since such phrase were clearly not instituted by Chazal, it would be obvious that it is asur to say them. Similarly, the gemara in Brachos 33b teel sof a certain chazzan who began chazaras hashatz “??-? ????? ?????? ?????? ?????? ?????? ??????? ???? ?????? ??????? ??????”. Rabbi Chanina waited until he finished Shmoneh Esreh, then took issue with him for saying so many praises of Hashem, as if those he said were all there was to say. However, if he had added other words of his own that weren’t inappropriate, R’ Chanina would not have been bothered to hear a nusach he had never heard before in his life. See also on 34a that R’ Eliezer had no problem with chazzanim adding and subtracting from Shmoneh Esreh at will, as long as they said the substantial parts of tefillah.
In short, all nuschaos of tefillah that have developped, Ashkenaz, Sefard, Moroccan, Yemenite, etc., are kosher. The question is only whether one has a right to change from the nusach that his ancestors davened.