My three favorites are all from laserdiscs:
John Fricke's for the Ultimate Edition of The Wizard of Oz. He did commentary for the DVD/Blu-ray releases, too, and some of his commentary is the same but it's interrupted with sound bites from others and not quite as effective on the DVD/Blu-ray.
Ron Haver for Criterion's Singin' in the Rain.
Miles Kreuger for Criterion's 1936 Show Boat. (I believe MGM re-used it for their Show Boat laserdisc box set, too.)
John Fricke's for the Ultimate Edition of The Wizard of Oz. He did commentary for the DVD/Blu-ray releases, too, and some of his commentary is the same but it's interrupted with sound bites from others and not quite as effective on the DVD/Blu-ray.
Ron Haver for Criterion's Singin' in the Rain.
Miles Kreuger for Criterion's 1936 Show Boat. (I believe MGM re-used it for their Show Boat laserdisc box set, too.)