I am quite fond of Britten's Serenade for Tenor, Horn and Strings. My favourite recording is with tenor Peter Pears, hornist Barry Tuckwell and Benjamin Britten himself conducting. You can be pretty sure that the tempos on this album are quite definitive.
A shocking document is a piano roll of Gustav Mahler playing the first movement of his 1st symphony. It is impossible to say whether or not he truly imagined the symphony as sounding the way he plays it on the piano, given the different mediums... but if so, then the entire world would have to reconsider their approach to his music, as the rubato on the roll is so incredibly free that it makes Chopin sound like Bach.