Sherlock: Your hands. He loved how much smaller they were than his and how the fit just perfectly in his as well. Sometimes when he is bored and not shooting his wall he will play with your fingers absentmindedly.
John: Your nose. He really doesn't have a reason but if he had to give a reason any at all he would say it's because he loved the way you looked when you would crinkle your nose is made you 100 times cuter in his opinion.
Mycroft: Your eyes. He loved how unusually bright thy were. He found them mesmerizing ever when the were red and teary. Sometimes you'll get frustrated because you will be talking and he is to distracted by your eyes and he never heard what you were actually saying.
Moriarty: Your lips. He loved how (plump/or thin) they were it made them extra kissable. He also loved how they looked when you twisted them into a smirk or a mischievous grin.
Greg: Your shortness. Your height made you even cuter because he could come up behind you and rest his chin on top of your head and it made you lighter which made you easy to carry on those days you were to lazy to walk and he had to carry you.