The Mobile Infantry by itself. It's so efficient and devastating that the Skinnies consider destroying one of their own skyscrapers to kill one soldier a good trade-off... And the MI agrees.
Best shown in the opening battle. About fifty of them attack a Skinny city with a spaceport, raze it to the ground, nuke the one starship that was there, and scare the Skinnies into ditching their alliance with the Bugs and join the Federation (the whole point of the attack), and suffer exactly one dead (plus a few wounded).
Rico's monologue about getting the shakes before every drop. He talks about how the psychologists worked him over, and declared that he wasn't scared, he was just eager to get into the action. He disagrees, stating that he's terrified every time. And yet, there is never any mention in the book of him hesitating to drop with his platoon. He steps into the Drop Pod and drops into hell with his men every time, despite his fear.
Everything Zim does. When he finally appears in combat, the author is forced to omit the name of the mysterious "best sergeant in the fleet," because by that point the reader already knows that if Zim goes into a tunnel filled with Bugs he'll get out safe, sound and with a Brain Bug captured.
Heinlein's Shout-Out and salute to Rodger Young, one of World War II's better-known Medal of Honor recipients, who died to save his squad.
Captain Yvette Deladrier does the impossible and stops a space ship to wait for some incoming troops. "If the Almighty ever needs someone's help to keep the stars in their courses, I know where He can look."
Rico taking out the first tanker bug they encounter using his old future-football moves to get onto the bug, then blasting a hole in the bug's back and throwing a grenade inside. It's topped later by Dizzy killing one by tossing a grenade into its open mouth.
The Bugs turn the aforementioned Klendathu drop into a win for them, killing 100,000 troopers in the first hour of combat. By the end of the battle, the human body count has risen to over 300,000. In addition, if you listen, during the battle you can hear a com intercept stating that the Bugs are overrunning the retrieval ships.
During the film's second battle sequence, an unfortunate trooper finds out the hard way that Bugs too know how to Kill It with Fire.
The bug's bombardment of the ships in orbit around their planet. The navy was informed that the bombardment would be random and light. Instead, the bugs demonstrate that they can easily shoot down, from the ground and with no computers to assist them, the majority of the fleet with ease.
The bugs in general continuously demonstrate that, in spite of humanity's arrogant assumption of superiority, the bugs are better fighters, have superior numbers, and their leaders have far superior intelligence, compared to the humans in this movie. And this is a species that demonstrates no technological advancement whatsoever, to the point that no explanation is even given for how they managed to attain space travel in the first place.
Zander's defiant, prophetic Last Words: "One day, someone like me is gonna kill you and your whole fucking race!" He might have been a tool for most of the movie but he died like a man.
Not to mention that before he is brought before the Brain Bug, he discretely hands a knife to Carmen so she might be able to buy herself a few seconds before Rico and his troopers arrive.
Also, earlier in the film when Zander and Carmen rescue the Roughnecks from the ambush at the outpost, Zander jumps out of the rescue boat almost as soon as it lands and starts laying down covering fire. Say what you will about his personality, but the guy is no coward.