What Siddur did Rashi use?

ב"ה Shalom Your question is a good historical question and I am not sure there is a clear answer for that. Rashi grew up in France but at the age of twenty, he went to study Torah in Mainz, Germany. There Rashi encountered many traditions from the Jewish centers of the world at the time especially from Babylonia and Eretz Yisrael. Rashi also had some distinct customs which are not in practice in the Ashkenazi community today.(see for example Sefer Hamanhig Vol. I. pg. 150) There is also a book called "Siddur Rashi" which isn't actually a siddur but rather a compilation of customs of Tefilla compiled by Rashi's students. (See חכמי צרפת הראשונים, of Professor Avraham Grossman, pgs. 130,158, 239) In the 1910 edition published in Berlin , the editor shows a great variety of sources for Rashi's customs, from the Geonim of Babylonia to contemporaries of Rashi from France and other countries. So unless you can find a historian who tells you otherwise, I'm not sure we can pinpoint from which Siddur Rashi used. All the best