You know, I've never actually really thought about this question much. But now that I do think about it, a couple stand out to me;
#51 just seems like a really epic story in the sense that it's within the final arc of the series and the whole thing with the governor and the defence of him just highlights how serious the Yeerk invasion of Earth had become;
The David trilogy, like some have said, specifically #21, as it's the turning point (although there were pretty much hints from the start he was a bit dubious) of the character and the short arc. (I cheered at the end of #22; he completely deserved everything he got.)
And I also can see why people like #26; I really like how K. A. Applegate showed the Animorphs' everyday lives, and how that could sometimes (or, did) intertwine with their mission(s) and the seriousness of what they were getting into. It also involves the Ellimist and Crayak, so it's also pretty epic in that way too. I also liked books early on in the series for that reason, as well; it hadn't become fully serious and as dangerous by that point, and they were still trying to live some semblance of normal lives. (Specifically #6 through to #14 - a bit of a stretch, I know.)
But through all of that, I can still appreciate practically every book, including all of the
Megamorphs (all of which I really love) and
Chronicles books because they all have good intentions about them, even though some aren't all that significant to the overall plot or are character development stories and explorations. It shouldn't all be about the end result, should it? Most of us know how the series ends, so why not go back and enjoy the ones we looked down upon before, or didn't really strike us on the first read-through?
