I hate to be contrary but you just can't beat the Queen of Blades, there's nothing more satisfying than hotkeying your hives and important buildings then covering your entire base with Overlords so your enemey can't see what you're doing. Build up your base and your force, any attack by another player will target the Overlords first, and they're expendable . . . then, when you're ready, there's nothing scarier than moving the whole Overlord-Horde slowly, menacingly towards your enemy base, your entire land army hidden beneath them. What does your opponent see? A zergling here, a hydralisk there? No way to know for sure what's coming.
Of course agaisnt the AI this strategy is a pointless waste of resources, and though it can work in a rush game it's really best saved for "no rush" but when it does happen it's pwnsome!
The closest the Protoss come is an invisible carrier swarm, and the best the Terrans can pull off is a mid-base siege tank drop. Both are effective but neither as totally jaw droppingly awesome as seeing the full force of the swarm creeping meaninfully towards your base.
Unless you play Brood War, then the Protoss just downright own because they can do all 3, but meh, none of my friends have Brood War so I never use it.
. . .
Oh wait, we're talking about Animorphs and battle morphs! Um . . . um . . . I would totally morph an infested Terran. I'd be a
blast at parties!