"Oh! The Things Fink Can Think!" by DrFaustusAU
"Broken Silence" by Natasha Saville
"Boys of Silence" NECA Figure, photo by Ed Speir IV
One thing the Bioshock series has done well is give you these armored beings (Big Daddies, Big Sisters, the Handymen, the Songbird) and not really explain who they used to be, or how the hell they ended up in such a dire circumstance. Same goes for the Boys of Silence, although given the context of where you encounter them (seriously, that entire level had me questioning my character's sanity, and whether or not everything had been a delusion up until that point) they just seem all the more strange and unsettling.
I'm really hoping the Songbird's history is explored, a la the Big Daddy in Bioshock 2, in a sequel or DLC pack someday. My second runner-up would be these creepy dudes.