Since it's a two-part question, I'm giving it a two-part answer.
Starting with the Marines.
The mission is to reestablish contact with an unresponsive corporate colony of a few hundred people. Latest intel indicates a xenomorph may be involved.
And who gets sent?
1 officer (Gorman), 1 NCOIC (Apone), 1 android (Bishop), 2 landing craft crew (Ferro & Spunkmeyer), 1 corpsman (Dietrich), 1 comm tech (Hudson), And 6 grunts (Hicks, Drake, Vasquez, Frost, Crowe, and Wierzbowski) Plus two civilians (Burke and Ripley).
This doesn't look like a cohesive grunt unit. This looks like either an advance party or a remain-behind element of a unit that's deployed elsewhere.
The company asked for a working party of Marines, and got a dozen of those who were doing make-work until they could return to their own units or muster out (Hudson is an admitted short-timer, others might have been).
In other words, nobody at higher gave a shit, and the Marines sent to do the job got screwed. They cowboyed up and did it, oorah. But they were screwed before they left. Everything that subsequently happened to them, from Ripley's briefing (see below) to the clusterfuck in the processing station, stems from that.
As a side note, the single greatest defeat the Marines have come as a result of obeying lawful but mind-bogglingly stupid orders. The fact that any of the Marines survive the processing station is because Drake and Vasquez both shrug and say, "fuck it. I'll take the ninja punch if we live." attitude.
On top of that, when ambushed, losing two levels of leadership, and at half the strength they started with, the Marines still managed to regroup and extract under fire.
It keeps on going from there. When Hudson notices that Dietrich and Apone are still alive, Vasquez insists on going back in to extract them. Vasquez just got pulled out of a nightmare that killed half her squad, watched her battle buddy melt in front of her eyes, and her first instinct is to go back and get those still alive. It takes Ripley, who was just proven right in the most horrible way possible, to convince her that they're beyond rescue. Her second choice is using nerve gas on the alien nest.
This sets the tone for every death in the dwindling party thereafter.
Yes, the Marines were defeated. But they all went down surrounded by the bodies of their enemies.
So that's the Marines.
Let's talk about Ripley.
What a lot of people forget about Ripley is that she's extremely practical. She was a blue-collar oil worker, a warrant officer on a mining ship. And that practicality serves her throughout the film.
When the movie opens, she's not helpless. She's got a serious case of PTSD and a bad case of being screwed over by the company because she was too busy staying alive to preserve evidence. But she's not stupid, entitled, or out of her depth.
The first impressions the Marines have of her is screaming at Bishop and throwing a tray across the galley for no apparent reason, then an absolute shit show of an intel briefing.
The grunts welcome useful briefings. And they can sail through boring and repetitive powerpoints easy enough. What they see from Ripley is a clearly traumatized civilian who is not prepared to give this talk at all, and who barely makes it three paragraphs before breaking down at them. So most of them regard
her with mild sympathy at best. Most go back to not giving a shit.
The second impression is what gives her moderate respect. Instead of sitting on her ass like Burke (which she could entirely get away with), she goes to Apone, points out that she's certified on their loading equipment, then offers to help out. When offered the loader, she proves her competence within a few moves and is welcomed to the working party. Civilians doing the scut work that they don't have to do AND they're competent enough to do it without having to teach them a 101 class? That gets you respected.
When on the planet, She doesn't try to prove herself to anyone. She doesn't offer strategic advice, just confirms or denies what she's asked about the alien evidence the squad finds. But she does ask a lot of the relevant questions that get missed by the others. She's the one who points out that using armor piercing weapons in a fusion powered processing station is a bad idea. (Gorman made a stupid decision in response to it, but she was still right.) She's the one who points out Hicks is currently in charge of the dwindling party. She's the one who asks how long before they can expect a rescue party.
Then, when the shit goes down, Ripley's the one who gets shit done. When Gorman freezes, she's the one that drives the APC through a wall to extract the Marines. She's the reason that Hicks, Hudson, and Vasquez got out alive, and it was only dumb luck that she didn't manage to get Drake out too. It took a while for her to stop, and then not until after she'd crushed an alien under her tires. But she's the one who took action when it was desperately needed.
That get-shit-done attitude carries Ripley through the whole damn movie.
The second they catch their breath, She's the one who says "take off and nuke the site from orbit. It's the only way to be sure."
After the dropship crashes and they're stranded? She spends the first moment of downtime she has to learn to use their remaining weapons to the best extent possible, insisting that Hicks teach her everything.
When Burke traps her and Newt in the infirmary with a facehugger, she keeps her cool. When breaking the glass and getting someone's attention on the security cameras doesn't work, she sets off the fire alarms instead of panicking.
Duct taping a flamethrower to an assault rifle sounds insane on paper. But it works for both her goals and methods. She barely has time to reload, let alone transition normally, so she makes sure she doesn't have to.
When the queen follows them onto the Sulaco, Ripley ignores the heavy weapons she's never seen and goes for the biggest weapon she's experienced with: the glorified forklift.
It's the scifi version of taking down an enemy commander with an e-tool.
So yeah, that's why Marines love the movie Aliens and the character Ripley. You gotta get into the Marine mindset and look a little deeper to realize exactly why, but it's there.