That's a very interesting question, and the result does not only evolve when one deciphers a text, but also when new inscriptions are found.
Thus, even though only one tablet was found outside of Crete before 1973, I would say the answer to your question is Linear A: there are 1427 Linear A documents with a total occurrence of 7362-7396 signs.
The linear A is a religious writing of the Minoan civilization. It is believed to be the origin of the Linear B, the most ancient written form of written Greek, used until the arrival of the alphabet.
The main difference with Linear B is that
- Linear A is not deciphered while Linear B is.
- If Linear A is pronounced similarly as Linear B, then it is unlikely to be Greek and could in fact be a language with different origins (possibly semitic).
I am giving this answer considering that, on the contrary to Mayan — previously given, Linear A is completely undeciphered while Maya script is actually deciphered although many inscriptions remain a mystery.
Of course you need to keep in mind that Linear A may answer your question only because many searches were made in ancient Greece (same goes with Egypt for instance), while there could be some unknown script, or one for which only a few tablets was found, for which we might find a much bigger corpus after all. I am aware that you asked about the largest known corpus of text, and I only mean to say that you might have bigger chances deciphering a mysterious script language by actually looking for more inscriptions of it, than you would have by studying the script that has the biggest corpus of text.