I was a bit too old by the time I found out about the two later series to bother, but my younger brother's description didn't stir much interest. I initially didn't care much for Animorphs either, until I realized how gory they were for young adult, and the characterization that came through, particular that this team of teenage superheroes actually suffered PTSD. Before PTSD became a household term. Back then it was known as Gulf War Syndrome, and no one really knew what it meant, or thought it was peculiar to the first Iraq war. But Applegate had just about every character who took part in the fighting for an extended period suffer as a result to some degree or other. There are very few authors willing to do that in my experience, unless they are so explicitly anti-violence that they force contrived pacifist solutions on the story, or make their protagonists awesomely recover in time to save the day. Over the course of the series, KAA just broke down the heroism of all the characters. The idealist was burned out and didn't care about anything by the end beyond getting his friends out intact (and ended up losing all the ones he was closest to). The most overtly heroic and courageous, had her most heroic ongoing practice go unnoticed, and ended up questioning her moral fitness to the point where she felt almost unworthy to survive. The most obviously good character, who was the voice of morality throughout the series betrayed the team at the end and a case could be made that she caused all the tragedy of the ending with her failure of nerve. The smartest one, with the keenest insight, who wasn't interested in the fight for its own sake or the greater cause, gets yanked away from his happy ending, and in the final PoV of the series, seems to realize that they've made a huge mistake from the very beginning. And the kid who was the closest to the protagonist and hero wins an empty victory that leaves him lost and broken.

I don't think I've ever read anything that had an ending where the heroes were so successful on paper and so utterly destroyed by it. Even stories where everyone gets killed in the end, there is usually some suggestion of a worthy sacrifice or achievement of peace in their fall. Not this.

Animorphs.
Rachel was my favorite, though I thought Tobias had the most interesting arc and stuff going on beneath the surface. BTW, those archetypes were combined I thought. Rachel was both the cute girl and the closest to a strong lead. Jake is the obvious choice for strong male lead, but he was more of a blank slate, IMO, for the reader to project themselves on, and for all that he was a decisive leader, tended to be kind of plot-driven and passive in his choices. Marco was the other three adjectives all rolled into one.
The 2nd & 3rd Megamorphs books, where they traveled through time were pretty neat, the first because dinosaurs and the second because I'm a history buff and a sucker for alternate history. Also the various Chronicles were all pretty awesome.
Anything involving the helmacrons, or Cassie's solo adventures.
I would have liked some sort of fake technobabble to handwave or lampshade the arbitrary limitations of the morphing power, since as it was, you get the strong impression that KAA just made it that way for writing convenience. As far as an actual plotline, I'd have liked to know who the big voices were who messed with Jake's dystopian future story, what happened to the shipwrecked Andalites, and for the Time Matrix to have played an actual role in the main story, rather than being confined to the peripheral outrigger stories. Introduced as it was, it seemed like a Chekov's Gun that never got fired. I spent much of the wrap-up part of the final book hoping it would pop up to undo the ending.
Until I read this post I didn't know they were a thing.

I got the impression that was supposed to be a sort of lady-or-the-tiger ending. Even if they won and made it out of that encounter alive, I don't think it would have mattered much, character-wise. Jake & Tobias were still screwed up beyond recovery, and Marco had not come through nearly as well as he initially presented his post-war lifestyle, so he probably wouldn't have had a happily every after either.

I get the impression that for as long as they kept on going, Jake would have been looking for a fight, Tobias either further withdrawing or going dark (I always thought there was the potential for someone really hard and cold there, who would be scary if he actually ever did break). And Marco would have gone along with either or both of them as he realized that everything he dreamed off getting was kind of empty and meaningless next to his wartime experiences. In short, if their suicide charge at the blade ship didn't kill them, they'd have kept charging ahead, figuratively speaking until they found something that would. By the time they emerged from the war, all that was left was the Dark Knight choice - die heroes, or live long enough to become the villains. At least according to the mores of Animorphs.