We don't have (and more's the pity!) R' Yehudah Hanassi's autograph copy of the Mishnah (well, okay, it's a controversial issue whether he actually wrote it or only arranged it orally), or Ravina and Rav Ashi's of the Gemara, etc.
But what is the oldest known manuscript of the Mishnah, Gemara, Mechilta, or any other part of Torah Shebaal Peh? (Or perhaps there are manuscripts of Geonic writings - responsa, Halachos Gedolos, or whatever - that predate any known copies of the foundational texts?)