Have to admit, Joss would be good, I'm a huge fan of Buffy/Angel and Firefly and I think he has the style and the wit to carry it off. To be fair, everything he's done, all the TV and film projects that he's prominently associated with have been his own creation. He's worked for years as a writer on X-Men for example, and no-one (to my knowledge) has accused him of Whedonising everything. He strikes me as someone who would be respectful to the source material.
Spielberg has got to the point where I will never suggest him for a film, he just sort of goes off and does his own thing. I doubt he'd be interested anyway.
Directors:
-Todd Holland?
-Joe Dante?
-Bryan Spicer?
Jo Dante's my favorite of the three, while he's done a lit of comedy he's got a few decent horrors in there as well. And I have to admit, I did love Gremlins.
While not horrible choices I don't think the other two really lend themselves naturally to an Animorphs film. They both have predominantly TV background, and while Spicer has worked on some good series in recent years, Holland has mainly comedies and a kids film on his resume.