Ooh, a Spirit Tracks thread! Me likey!
I'm gonna go with Warp Phantoms, not only for when you possess them, but when they're your enemies as well. Obviously warping is a very handy ability to have, and I made good use of it in my playthroughs. But even more than that, the reason I like them is for the element of fear they instill in me. I'm walking along trying to mind my own business, this weird eyeball thing strolls around the corner and beeps at me, and then this huge lug just pops up RIGHT BEHIND ME!
Warp Phantoms: the (not-so) Slender Men of Spirit Tracks.