The Wizard of Oz (1939)

1988 Criterion LaserDisc [CC1159L]: Heavy noise reduction. The most muffled and distorted rendition of the mono by far. The channel in the analogue track with the soundtrack uses the same mastering.

1989 MGM LaserDisc [ML101656]: The digital channel with the film's soundtrack sounds excellent and is easily my favourite among these. While I don't think it's completely free of manipulation (frequencies above 8 kHz are de-emphasised), it runs circles around every later release. At 1:39:20 (equivalent UHD timecode), the first word in Dorothy's "There's no place like home" is truncated. The analogue track shares the same mastering. 

1989 MGM Hi-Fi VHS: Like the 1989 LaserDisc, but more brightly EQed. Not pictured below.

1991 MGM LaserDisc [ML101656]: Identical to the 1989 LaserDisc.

1993 MGM LaserDisc [ML103990] - Ultimate Oz: Less noise removal than the Criterion, but significantly more than the 1989 LaserDisc. Some bass removal too.

1997 R1 MGM DVD: Like the Ultimate Oz LaserDisc but with some limiting.

2005 R1 Warner DVD - Special Edition: Based on the fluctuations in hiss visible on top of ~every utterance of dialogue, I think this is a new mono mixdown created from 'restored' (NRed, de-crackled, de-essed, EQed) dialogue and music/effects stems. Normally I'd arrive at this conclusion by finding sync discrepancies between the dialogue and music/effects, but I can't find any here - which just means that someone did a competent job syncing the dialogue and music/effects to match the theatrical mix. At 9:50 (UHD timecode), Dorothy says (what sounds to me like), "Oh don't!" The take itself isn't different, but the first two words are edited; in every previous release she says (again, what sounds to me like), "Oh To, oh don't!" Some of the background effects as she fiddles with the basket in the next 1.5 seconds are also subtly different. Fidelity-wise, this track sometimes sounds better than the Ultimate Oz mastering and sometimes worse - it varies from scene to scene, but I think they're grossly similar.

2009 Warner Blu-ray: Like the 2005 Warner DVD.

So much fuss has been made online (usually by the same people) about the edited Toto line. I think it's completely inconsequential relative to the horrendous amount of hiss and bass reduction on every modern release. (I encounter small editing mistakes like this in so many tracks that I usually don't even bother mentioning them here.) The 5.1 mix sounds the same across all releases and has the Toto error, but more importantly it has even more noise reduction than every mono track above other than perhaps the Criterion.