My favourite thing...the fact that I read Animorphs when I was 11-15 years old (except for 50,51,52 and others) and it reminds me when I was young, handsome, strong, and I had nothing to do from morning to night....
Ok, Seriously : I like the general plot of the series, especially the morphing power and the aliens, particularly the Andalite, they are so incredibile with their appearance. And, obviously, the covers....very well-designed. And the names...I like names like Elfangor, Arbron, Esgarrouth, etc., they sound epic (and some are probably inspired by Tolkien)