Sherlock:
The first time you tagged along on one of his cases he did his best to impress you. Though he did come across as an arrogant jerk, you'd be lying if you said he wasn't bloody brilliant. Sherlock solves the entire crime by himself in only a matter of minutes and you were blown away by his detective skills and found yourself falling even more in love with the detective.
John:
You and John were taking the subway to meet up with some friends when a guy just collapsed on the ground. Without a second thought John was by the man's side. You were impressed, to say the least, when John saved the guy by performing CPR. By the time the ambulance had arrived the guy had gained conscious and was finally stable. You couldn't help but to adore John's braveness.
Moriarty:
It was your first real date and James did his best to impress you by being a gentleman the entire night. You had already seen his tough, and kinda scary, side so he wanted to show you that he did care enough to put some effort in being gentle and polite towards you. But only you. It wasn't a one time thing either. He always made sure you understood just how much he loved you.
Greg:
On your two months anniversary Greg wanted to cook you dinner. You were surprised by how delicious the food was and even more impressed as Greg had bought your favorite beverage. He knew you and he had listened to what you had told him about yourself. That's how he impressed you the most.
Mycroft:
He impressed you when he told you your own story. He knew all about you. everything to your favorite color to what you did for a living.