Just the fact that when all is said and done, the Losers accomplished everything they did in the book without help from the Ritual of Chud or The Turtle.
Unlike his book counterpart, Adrian Mellon is very much Defiant to the End, insulting his attackers and giving them a piece of his mind, up until they pitch him over the bridge and into the river.
Unlike Bev and Ben, Bill makes his way out of Pennywises personal hell for him without any outside help. He drowns a Georgie designed to torment him with guilt, and when his younger self tries to shoot him with a cattle gun, it doesnt work simply because he doesnt believe he deserves it, finally coming to terms with nearly three decades' worth of self-loathing and guilt. He then turns the very functional gun around on the monster and escapes.
Bill: Just because you didn't want to play on a rainy day? No, that doesn't make you a bad person. You were the best big brother, and he loved you very much. And that (takes the gun from his younger self and aims it at his forehead) does not make it your fault (shoots Pennywise in the forehead).
Richie killing Henry in the library after he attacked Mike. Also Eddie earlier stabbing Henry in the gut with his own knife after Henry jumped him in the bathroom and stabbed him in the face.
Before all that when Henry attacked Mike, there was Mike flipping Henry onto a display case.
Despite suffering a humiliating defeat to children decades earlier, Pennywise hasnt lost his edge. He is still capable of manipulating both children and adults before trapping and devouring them.
Eddie nearly succeeded in ending Pennywise before their final confrontation. After conjuring up the nightmarish scenario in the pharmacy basement, Leper Pennywise tries to choke Eddie into submission, instead, Eddie, who had been adamant on leaving and just ignoring the clown's existence, managed to turn the tables and strangle Pennywise instead. Eddie would have won if Pennywise didn't vomit onto Eddie to escape.
The final battle between the Losers and Pennywise.
Bev overcoming the deathtrap Pennywise left her in, motivated by Ben's seemingly dying declaration of love and her own anger at Pennywise, by kicking the stall door open right in Pennywise's face.
Even better is that Pennywise is once again taking the form of Bev's father to mock her as he's slamming on the stall door, leading to this:
Pennywise: Are you still daddy's little girl?
Beverly: Not anymore! (kicks door hard enough to cut off Pennywise's fingers)
Richie chucking rocks at Pennywise's head, giving the other Losers ample time to move and find some cover. All throughout, he insults the Monster Clown left and right, even using Pennywise's earlier taunt of playing "Truth or Dare" against him, not even scared anymore of his sexuality being thrown out into the open. Although he gets caught in the Deadlights a few minutes later, it's still an awesome enough moment for Richie.
Richie: HEY, FUCKFACE! You wanna play Truth or Dare? Here's a truth: you're a sloppy bitch!! Yeah, that's right, let's dance! Yipee-ki-yay, motherfu - (gets caught in the Deadlights)
The fact that Pennywise had to use the Deadlights in this moment, which was shown to be a last resort when used on Beverly in the first film, actually emphasises the awesomeness. Richie earned it by not being afraid.
The Losers defeat him once and for all by forcing him to believe he is small, which causes Pennywise to shrivel up into a powerless shell before the Losers pull his heart out and destroy him.
Mike in particular is the first to get under the clown's skin, saying in a perfectly calm tone:
Pennywise: Me, small?! (Evil Laugh)I am the eater of WORLDS! Mike: Not to us, you're not. You're just a clown. Pennywise:(is shocked silent, staring off into the distance)
The Losers are so pissed at Pennywise by this point that their insults are filled to the brim with Precision F Strikes.
Beverly: YOU'RE A FUCKING BULLY!
Ben:YOU'RE—A MIMIC! A-A-A MIMIC! A MOTHERFUCKIN' STUPID MUMMY!!!!!!
Bill: IMPOSTOR!! FUCK YOOOOOOU!!!
Mike: YOU'RE JUST A FUCKIN' CLOWN!
Richie: A CLOWN! A DUMB FUCKING CLOOOWN!!!
Richie pretty much rips off one of Pennywise's spider legs out of anger.
Eddie managing to use Clap Your Hands If You Believe and throwing Beverly's spear into Pennywise's throat to save Richie from the Deadlights. Unfortunately, this victory is shortlived as Eddie is stabbed afterwards. But this moment was when the Losers realized that they simply needed that belief to destroy Pennywise forever.
Eddie: (gripping Bev's spear) This ... kills monsters ... if you believe it does. (more confidently) If you believe it does. BEEP-BEEP, MOTHERFUCKER!!!!! (hurls the spear into Pennywise's maw)