From one Animorphs fan to another, is there a series comparable to Animorphs that you could recommend?
TBH, I don't know of any that are super similar. Animorphs is such a product of its time and its author(s) that even series that directly competed (Goosebumps, Dinotopia) look nothing like it. However.
Series that do cool things with first-person narration like Animorphs:
Queen's Thief by Megan Whalen Turner. Set in a fictional version of ancient Greece/Turkey, the series follows one protagonist (primarily a thief, but hobnobs with queens) through a handful of different narrators who see him as a hero, a villain, a loser, a genius, a traitor, a savior... And they're all kind of right.
Pendragon by D.J. MacHale. The world's most relatable teenager narrates adventures across the multiverse. It's notable for its overt grappling with questions of ethics during war — at one point the narrator has to decide whether to kill 30 innocent people to keep nuclear weapons out of Nazi hands.
Otherbound by Corinne Duyvis. Truly a book about "we live in a society," only the twist is that the two different protagonists live in two different universes with two different definitions of "disability"... and then they develop a psychic connection.
Children's books that will ruin your life (/pos) like Animorphs:
The Nest by Kenneth Oppel. A heartbreaking and horrifying story about loss, fear, being disbelieved by adults, being overshadowed by a new baby, and the dawning realization that grown-ups can't fix any of the things that are most fundamentally wrong with the world.
Lewis Barnavelt by John Bellairs. The series that shows that children's horror does not have to be cozy, when it could instead be paranoid and atmospheric and disturbing.
Feed by M.T. Anderson. This book came out in 2002 but could have been written last year, all about how advertising is eating the lives of contemporary teens.
Books with many of the plots/structures that make Animorphs good:
Tomorrow, When the War Began by John Marsden. Thank you @zarohk for recommending this series — it really is the closest to Animorphs I've ever found. A group of teens is camping in the Outback when the alien invasion takes their town, leaving them to decide how much violence they're willing to do in order to try and free their parents.
Endling by Katherine Applegate. Do I cheat by including this? Very well, then I cheat. An unapologetically disturbing premise (an "endling" is the last animal in an otherwise extinct species) gets softened over time by the power of friendship and screwball comedy, without losing sight of the horror.
Die by Kieron Gillen, or The Power Fantasy (Gillen), or The Nice House by the Lake by James Tynon IV. All are comics, not novels, so I'm grouping them at the end. But all have the common thread of "queer found family gradually turns toxic while dealing with dark magical adventures," and all remind me of Animorphs' uplifting yet disturbing codependency between the protagonists.
If other people have recs, please weigh in!