Son Goku gets his in the Kami-sama arc. After suffering their first really crushing defeat, the Sanzo-ikkou is half-dead and out of options, yet they have no other choice but to go after Kami-sama again, since he stole Sanzo's scripture. Knowing there's no way they can win with that kind of attitude, and hating to see his companions so out of it, Goku forces them to sit down and play mahjongg. And he wins. Repeatedly. They play for an entire day because Gojyo, Sanzo and even Hakkai are so incredibly pissed off that Goku, who has never, ever, beaten any of them, in mahjongg, cards or even rock-paper-scissors, keeps winning with Sanzo's discarded tiles. And never mentions food, even though "I'm hungry!" is almost his Catch Phrase. Finally, Sanzo can't take it anymore and moves to go after Kami-sama alone, when Goku asks him if he's really all right with leaving the table a loser. According to Goku, he's not winning because he's playing strong; he's winning because they're playing weak. And he goes on to give a passionate speech, about how they have their own way of doing things and how he's never, ever going to lose again. He runs out of things to say after a bit, but he gets his point across and he gets his desired results. It's a significant moment in the series that really highlights how far Goku has come on the journey west, and that he's beginning to grow up; this was the first time fans could begin to take him seriously as a character in his own right, not just an Expy of every other stupidShōnen protagonist with a big appetite.
The entire scene is a giant Shut Up, Hannibal! moment delivered to all of the ikkou's (and especially Sanzo's) inner demons that were brought out by Kami's Hannibal Lecture. Which makes it a Shut Up, Hannibal! moment that still managed to be badass even without Hannibal there...
Sanzo's fight with many youkai in an underground tunnel in the Reload manga. Provides some Character Development in terms of how Sanzo and Hazel differ in their treatment of youkai and battle philosophy, and an aversion of Bottomless Magazines: In a scenario that reminds him of his years spent hunting for the Seiten sutra, Sanzo is down to 4 shots and 5 opponents and finishes off at least two of the attackers with a katana that another youkai dropped. It's a rare glimpse of him fighting without his gun and his level of Badass is undiminished.
Could be considered a CMOA for Minakura's storytelling as well. It is the first time since he fully accepted his responsibility as a Sanzo priest and stopped tearing himself apart that he has truly been alone fighting for his life, just like he was for most of his time searching for the Seiten sutra. That experience broke him as a kid, and he only really pulled himself together a little before meeting Goku. This was a scene that asked the question "has he really changed at all or is he just keeping up a front around the people he travels with so as not to appear weak?" and Minekura manages to answer a resounding yes with just two panels: the first a flashback panel to a young Sanzo with his snubnosed pistol pointing at his own temple, and the next a picture of current Sanzo pointing the gun at the audience.
Then he goes back to the town, where things were just on the point of getting ugly between the inhabitants and the others, fires the one bullet he saved into the air to get the crowd's attention, announces WE ARE LEAVING RIGHT NOW, and nobody dares to do anything to him or to the rest of the ikkou.
There's another awesome moment of character development/character crystallization in Reload, shortly after the Sanzo ikkou first encounter Hazel. Hazel, who believes that youkai lives are worth less than human lives, has a long, honeyed speech where he coaxes a little youkai boy to let himself be killed to save a human child's life. The other members of the ikkou look in desperation to Sanzo to intercede. Just as Gat is about to kill the little boy, Sanzo says to him, "You're shaking. Why are you scared?" "B-because I want to live." "Louder." "I want to live!" No pretty words, no complex rationalizations. Just the truth.
The ending of the Reload manga has the previously separated ikkou rejoin Sanzo, just in time to keep him from getting erased by Ukoku's Muten Sutra. With Hakkai healing Sanzo's injuries, Goku and Gojyo attacking Ukoku from two angles at once, and Gat and Hazel temporarily drawing Ukoku's attention, Sanzo gets off one shot Ukoku can't fully dodge, putting out his right eye! Not fatal, but far more damage than anybody else has been able inflict on Ukoku. Fired, for some poetic justice, from Gat's gun (which is about 3 times the size of Sanzo's S&W). And then everyone collapses on the ground bleeding and giggling and playing Jan-Ken-Pon to decide who has to drag The Luggage (Sanzo) to Jeep. The climax being their first battle directly against Ukoku has a wonderful symmetry with the end of the Gensomaden manga, where the ikkou defeat Ukoku's disciple Kami-sama and it's first confirmed that Ukoku is Nii Jienyi.
Goku, after nearly drowning, climbing a cliff, wandering around in the woods with a kid, fighting off a whole clan of yokai, destroying the entrance to a cave to save an entire village, and having not eaten that day, comes face to face with a brainwashed Kougaiji, who gets a couple shots in while Goku attempts their usual banter. Confused and exhausted, Goku gets the shit beat out of him for a while, until he realizes that he's waiting for the others to come save him. He decides that is seriously un-cool, and turns the tides of the battle. You only realize how much it's taking out of him, when the others finally do show up, and he only remains standing until they literally tag him out of the fight. Then without hesitation he collapses backwards and passes out.
The kid he was with also deserves mention. When there is sign of trouble Goku collapses the entrance to the cave he and his village were hiding in to protect everyone living there. The kid, despite the protests of his mother, starts clearing the rocks from the inside of the cave, so that he can go help Goku out, because he'd helped him so much. And he manages to convince the entire village, who had no idea who this Goku guy was who'd trapped them in there, to help.
Gojyo, faced with the possible death of his best friend, who is also the first person since his brother he ever really cared about, and the only one who never abandoned him, tells Hazel not to touch or try his bringing back to life thing on Hakkai, because Hakkai wouldn't want that. Gojyo, with all his abandonment issues, chooses to let Hakkai die rather than turn him into something (else) he'd hate. Then he manages to revive him anyways.
Hakkai managing to fight on par with Seiten Taisei probably had something to do with the element of surprise, but it was pretty badass anyways. He would have even managed a fatal blow (not that it would have killed Goku in that state) if Gojyo hadn't stopped him from going too far. Also, DEM VINES.
Tenpou punching the living shit out of Li Touten, for trying to get Kenren fired and sexually harassing Tenpou (no kidding—Li Touten comments on how pretty Tenpou's face is and then speculates on his preferred sexual positions). The cold fury radiating from Tenpou is the first glimpse we get of just how dangerous he really is.
At the Emperor's birthday celebration, Tenpou and Kenren getting in a fight with some soldiers on Goku's behalf is pretty damn awesome already. Then Konzen shows up, gives Goku a noogie for running off, and tells the entire assembly that all three (Goku, Kenren, and Tenpou) are his responsibility and WE'RE LEAVING RIGHT NOW. And the three of them fall in behind him without a word as he stalks off. It's the first time Konzen displays this kind of charismatic authority and conviction and it becomes much more apparent in his reincarnation as Sanzo.
(Doubly awesome in that it shows Konzen finally shaking off his boredom and frustration and committing to a cause - getting to the root of the corruption in the Heavens.)
Kenren vs Bear Nataku. After temporarily subduing the monstrous bear, he fights his commanding officer, supposedly to the death. Then, when Bear Nataku gets back up and nearly kills Gojun from behind, he kicks the dragon king down a hole, saving his life and taking the blow. It becomes even more badass when it makes you realize that Kenren wasn't fighting Gojun to the death. He was both stalling for time for the others to escape and giving Gojun deniability at being complicit in the rebellion. Then, after taking a fatal bite he, while bleeding to death, manages to convince Bear Nataku to commit his own act of rebellion, and instead of leaving evidence (in the form of some of Kenren's body parts) that the biological experiments were conducted by the Gaiden-ikkou and not Li Toten, Bear Nataku at Kenren's insistence eats the god entirely. And reveals himself to be at least partially sentient by crying as he does so. A fitting tribute to someone so dedicated to doing whatever he liked. This is also the first time Kenren showed Gojyo's ability to read people and empathize with them. Gojun dropped his weapon down a shaft (something that must be like pulling nails to an experienced soldier) so that he would have a free hand to salute Kenren after this. Thats how badass it was.
Konzen, who is a bureaucrat and completely useless at fighting, still managing to be useful by protecting Goku with his body. Doubles as crowning moment of heartwarming, especially when he jumps down an elevator shaft to catch the kid, getting horrible burns on his hands from the cable he grabbed, and refuses to let go to save himself. Note that Konzen is not strong and Goku weighs 100K more than he should because of the chains and limiter's put on him.
Tenpou making a list of things he still needs to do in his head as he fights what he knows to be his final battle. He's pretty much singlehandedly fighting the armies of heaven to distract from Konzen and Goku, and he is making notes in his head about how he needs to buy more cigarettes (which are thematically associated with life). It's not that he's in denial that he's about to die, but more an acknowledgement that even though he's willing to, and he knew the odds when he made the decision to fight, he still didn't come here with the intention of dying.