The very fact that Eli Wallach came out of filming alive after all the dangers and misfortunes he faced filming. He survived accidental poisoning and strangulation for this one role and he made it out in more or less one piece.
Tuco getting revenge on Corporal Wallace by throwing him off the train with him, killing him and then severing the cuffs via an oncoming train. After the brutal beating he received at the hands of the corporal, you're pretty much cheering for Tuco at this point.
Blondie's scathing remark to Tuco when the latter threatened to kill him after Blondie found out the name on the grave where the gold is buried: "If you [kill me], you'll always be poor, just like the greasy rat that you are." Ouch.
So Tuco is by the blistered Blondie's bedside, desperately begging him to tell him the name on the grave in which the gold is buried. Blondie beckons him closer... and throws coffee in the bandit's face. And then to stop Tuco from doing or saying anything violent, proceeds to shut him up with just a few words.
Blondie: I'll... sleep better... knowing... my... good friend... is by my side... to protect me.
Having been left behind by Blondie in the middle of nowhere, Tuco makes it back to town by having to walk day and night through a scorching desert, with no horse and no supplies, through sheer force of willpower alone. We see him at the end of his journey and he's dehydrated and absolutely exhausted. You can only imagine what he had to go through to finally reach town.
Tuco's Establishing Character Moment, where he is accosted by a trio of Bounty Hunters. The bounty hunters kick down the door of the saloon where he's dining. We hear gunshots and see Tuco dive out the window with a pistol in one hand and a chicken leg in the other. As Tuco flees, the camera pans to the tavern's entrance where we see two dead bounty hunters and the third seriously wounded.
Every time the words (You see, in this world, there's two kinds of people my friend...) and whatever follows next but especially after the finale:
Blondie: Those with loaded guns and those who dig. You dig..