Hmm. . .I must confess that Valenti's playing on the performance David K. mentions didn't impress me as much as flutist William Kincaid's, whose pure tone and beautiful phrasing is really a must-hear.
The really astonishing 5th Brandenburg that I heard recently was Lenny & the New York Phil. from the late 50's with Lenny playing the harpsichord part himself. His cadenza is truly impressive, hardly a missed note and skillfully built up from beginning to end. I've no idea what he was playing on, since the recording pre-dates his own harpsichord from Dowd by quite some years. The overall performance is really quite nice--relaxed but not stodgy tempi and none of the exaggerated ritardandoes that Stokowski puts in his. It struck me that Bernstein had a good understanding of the rhythmic life of Baroque music--something we sort of take for granted today but was not necessarily the norm in 1959 (or thereabouts).
The performance is on a multi-volume historic Bernstein set that the New York Phil. put out recently, I'm not sure if any of the disks are available individually.