As you might have figured out, Sergio Leone's films got the shaft not only when they were originally released, but also in our more enlightened days.
In this list, I will concentrate on the best available sources for the sound and video. This list will be updated when new sources are released.
The Dollars/ Man with no name Trilogy:
A Fistful of Dollars (1964):
Video: While the Italian restoration is commendable, the Germans took it one step forward using the AVC codec. However, the opening titles are in German.
Sound: The Italians included the original English mono in lossy AC3, while the Germans included it in lossless DTS.
Top choice: Fan cut using the opening titles from the MGM BD and the German video for the rest of the film. Such a cut is unavailable at the moment.
Avoid: The MGM BD. The video is DNR'd, EE'd, you name it.
For a Few Dollars More (1965):
Video: The Germans used more than one source to create an uncut version, the only one in existence on home video. But for some reason, they included a newly made opening card in German which replaced the original English one.
Sound: The Germans once more include the original English mono in lossless DTS.
Top choice: Fan cut which uses the opening card from the MGM BD and the German video for the rest of the film. It is available on a certain paradoxical forum.
Avoid: The MGM BD. The video is DNR'd, EE'd, you name it. The print used was also cut by 50 seconds.
The Good, The Bad and the Ugly (1966):
Video: The Italian BD has better colors while the 4K regrade by kk650 has better detail.
Sound: None of the BD's offer the original English mono at all, not even in lossy AC3. the only way to get the original English mono is from the LD synched by TheHutt.
Top choice: No top choice, the Italian BD/kk650's regrade with the LD mono soundtrack are the top 2, and as far as I'm concerned, the only ways to watch the film at the moment. The regrade is available at a certain paradoxical forum.
Avoid: The first MGM BD. The video is DNR'd, EE'd, you name it. Also, no original mono.
The Once Upon a Time/"America" Trilogy:
Once Upon a Time in the West (1968):
Here is where things get slightly more tricky. Changes in this film were mostly made during the 1985 restoration due to ignorance by the restoration team. The "rising" scene was put back into the film, which removes the edge of the tavern scene, and the finale music was replaced with "Farewell to Cheyenne". Since this was the most common version for the last 30 years, Paramount would have gotten quite a backlash for making changes to the film's "original" structure. The German and French TV versions should be the real DC's and have the original finale music intact.
Video: Paramount's BD has no contestants like MGM had in the Dollars trilogy. It's pretty good, but has the "rising" scene which should be cut. There is an Italian BD, but it might be a port.
Sound: While I commend Paramount for including the original mono soundtrack (Albeit in Lossy AC3), the music during the end credits is still wrong.
Top choice: None at the moment, but a fan cut is being worked on, which uses the Paramount BD as the main source for the video, and the LD as the main source for the audio.
Avoid: Nothing as yet.
Duck, You Sucker/Once Upon a Time in The Revolution/ A Fistful of Dynamite (1972):
Video: Italian BD is the best HD release at the moment, and I have heard no real complaints so far. However, it lacks English subtitles which the Spanish BD has. Both use the same master, but the Spanish BD uses the inferior MPEG2 codec and is at 24 FPS rather than 23.976
Sound: Italian BD includes the original English mono soundtrack in lossless DTS while the Spanish BD includes it in lossy AC3. The mono mix is presented in 2 channel dual mono on both releases. However, there is a lack of synchronization between the channels. The last line of the film is delivered in Italian for some reason, as well.
Top choice: A fan cut using the Italian video, fixing the DTS mono and including the subtitles from the Spanish BD. I fixed the English mono myself, but it is not perfect, as the final line has a slight hiss, as the source was different. However, anyone who has the Italian BD video can certainly use this.
Avoid: The Spanish BD. Worse encoding of the same master as the Italian and lossy audio.
Once Upon a Time in America (1984):
Video: The old (2011) Warner BD isn't bad, but when compared to the Italian extended edition, it's showing less detail and image information than the Italian BD. However, the Italian BD compresses more than 4 hours of film with 3 Lossless audio options (2 of them are the Italian dub) on one BD50. At 17 MBPS VC-1, it walks a fine line on the compression issue. The video is also yellow tinted and the saturation lowered by at least 50%. The extended Warner BD is from the same master as the Italian BD, so the tinting and saturation problems apply. However, it is slightly better encoded with the AVC codec at 15 MBPS. The original cut provided is the exact same Disc released in 2011.
Sound: All releases offer the English soundtrack in 5.1, and while I don't have any issues with it, it would have been nice to have the original mono present. The LD could be used to preserve it.
Top choice: None at the moment. A regrade of the extended BD to match the colors of the first BD would be the best we will ever get in the foreseeable future. Such a regrade has been undertaken by kk650, and it is available through a certain paradoxical forum.
Avoid: The Italian BD. It's has worse encoding than Warner, and it is the same master.