80s Spielberg would have nailed it, yeah, but these days he's a little hit-and-miss. It'd definitely have to be one of his more intense flicks, not all Munich or anything, but you'd probably want to stay Jurassic Park and above I'd think, in terms of all that.
Honestly I think the best bet, if this were actually happening, would be a relative unknown. Someone with a vision, someone with one or two critically-acclaimed movies under their belt, having secured final-cut with the studio, but still without the baggage of some big-name director. Sort of like Rian Johnson with Looper, getting the Star Wars VIII gig, or I suppose Gareth Edwards getting Godzilla from his little indie Monsters.
As for Whedon, he generally only directs stuff that he writes, and I don't think he'd be the type that would want to adapt something like Animorphs. Much as I'm a fan, he's just not really right for it anyway, all of his stuff has this sort of whimsical idealistic/optimistic-in-the-face-of-all-odds quality to it, gritty & paranoid/cynical ain't really his thing.
EDIT: Come to think of it, Katherine "The Hurt Locker/Zero Dark Thirty" Bigelow might even be a rad way to go. She's pretty in-your-face and visceral, but with a sort of playful snarky sense of humor to things too. She'd certainly handle the war-movie prerequisite no problem. Has some experience with crazy high-concept sci-fi, Strange Days is an amazing flick.
