Scott- Gag- you bought it to prevent your parents from hearing your name screams during hardcore sex.
Stiles- Pillow. He would sometimes fuck you into the pillow, and, any noise you made, would be muffled by the material.
Derek- Grips your throat- it's dominating and dangerous, and makes you mewl as he fucks you up against the wall.
Jackson- Hand- when he's thrusting into you from the side of from behind, he puts his hand over your mouth to muffle your groans
Isaac- His cock. It's big, and you choke around it nearly every time, to his pleasure. He wouldn't gag you during sex, because you both enjoyed the noises.