My first book was #6, and that one got me hooked immediately. I didn't read #1 until after reading five or six other books, all from around the beginning of the series, and I think a lot of people whose first Animorphs book was #1 don't realize that Jake, Rachel, Marco, Cassie, and Tobias never go into detail about how Elfangor was killed. They'll start to reach that point in their "story thus far" debrief, but then immediately decide they don't want to think about it.
I'm thinking of making an online graphic novel version of #6...