Given the age and popularity of The Good, The Bad, and the Ugly, I don' think there is a lot to add other than an angle I'm pretty sure most people aren't aware of: the influence of the Spanish Civil War on the film.
Yes, it is well known that the climactic three-way duel was filmed in cemetery set aside for war dead, but there are enough features that - knowing more about the conflict than I did when I last watched the film - influence is very clear.
Perhaps the most obvious example is the way the war it portrayed. The armies on both sides in what was at the time the Far West were little more than volunteer bands, not well-equipped and disciplined armies. Artillery was scarce, and the war was waged as a series of raids. Static, attritional combat was simply impossible to sustain.
By contrast, Spain did have sectors of the front that were located in the wilderness but that nevertheless saw continuous low-grade fighting. The bridge sequence in particular is instructive. The Union fortifications are extensive and rely extensively on sandbags - unknown in the American Civil War, but common in Spain.
When Tuco and Blondie tell The Captain they wish to volunteer, his incredulous reaction is also instructive. Apart from the early months of the war, when volunteers flocked to Anarchist battalions or joined the Falange, both armies in Spain rapidly settled into conscription as the primary means of recruitment.
By contrast, conscripts made up only a tiny portion of the respective armies in the American Civil War, and certainly none would have been sent to a place where desertion would be so easy.
Though they represented the government, the Union troops are clearly modeled on the Nationalists, with strict discipline in dress and movement and it's not much of a stretch that the "blue coats" were seen as Blue Shirts (Falange) to the Spanish crew, who portrayed them accordingly.
Probably the most obvious example is the backstory given to Tuco when he meets his brother. It is imply not credible that someone growing up in the west would have no options other than banditry or the priesthood. This was an era of tremendous population movement, and new settlements were emerging all along the frontier. Sergio Leone's imagining of the American west as dilapidated and forlorn is iconic, but also inaccurate.
It is, however, what was going on in rural Spain during that time period. Deprived of opportunity, young people flocked to the cities to find factory work, and many were radicalized by Anarchism and Communism. That simply was not true in America.
There are other tidbits of course, such as random indirect-fire shelling (unknown in the US, common in Spain) and of course the rope-wrapped wine bottles in place of whiskey or beer.
It's still a great film, and seeing the American Civil War through Spanish lenses provides an interesting take on the conflict.
And the music is outstanding.