Off the top of my head, here are some contenders, all of which indicate my interest in political history:
a) Cassius Dio, Roman History
http://en.wikipedia.org/wiki/Cassius_DioPhotius' appraisal:
http://www.tertullian.org/fathers/photi ... eca.htm#71b) Dexippus, Skythika
http://en.wikipedia.org/wiki/DexippusPhotius' appraisal:
http://www.tertullian.org/fathers/photi ... eca.htm#82 (search for Dexippus, link broken)
c) Diodorus, Library of History (EDIT: I meant to say the lost books 6-10, and esp. here 21-40).
http://en.wikipedia.org/wiki/DiodorusPhtotius' appraisal:
http://www.tertullian.org/fathers/photi ... eca.htm#82 (search for Diodorus, link broken)
Note that Photius had all of these texts in the 9th century. If I had to choose one, it would be most agonizing indeed. I agree that none of these selections can be said to constitute high literature, but the survival of any of these texts would help us quite a bit. Dio would greatly add to the the late Republican period, as well as the first to early third centuries, while Dexippus would greatly aid us in understanding the poorly documented third century, and finally Diodorus would give us a comprehensive view of the Hellenistic period, which is so poorly documented. I suppose Dio would give us the greatly help, with Diodorus and Dexippus following respectively. So my choice would be Dio.
ἀλλ' ἔγωγε ἐξ αὐτῶν τούτων μᾶλλον αὐτὸν τεθαύμακα, ὅτι ἔν τε ἀλλοκότοις καὶ ἐν ἐξαισίοις πράγμασι αὐτός τε διεγένετο καὶ τὴν ἀρχὴν διεσώσατο. Dio LXXII 36.3