The first one took the bribe. Mycroft had no problems helping him find a new home, a new job and presumably a new life far from anyone named Holmes. Sherlock deleted him and Mycroft no longer even remembered his name.
Susan had been nice. Really nice. Really, really, too incredibly nice. She was always smiling. When Sherlock had called her an idiot who was too stupid to see that her girlfriend was not only using her for free dinners, but had a lover, and that said lover was most definitely a man, her smile wavered only for a second. Then it came back in full force and she gave Sherlock a quick but firm hug from behind, called him a "silly billy" and told him that he was very cute when he was "being all mean". She found her belongings on the sidewalk. It's where they landed after he threw them out the window.
The one that would go through eternity known only as "that Spanish fellow" lasted longer than most. He was quiet, ate out, and actually enjoyed the violin. The occasional explosion in the kitchen or gunshot in the bedroom didn't seem to bother him overly much. It was a shame about that eyeball. Sherlock was never quite certain how it got into the microwave with the left over Saag Paneer, but he was very sure that if something as small as that bothered him so, he probably shouldn't be living with the world's newest, and well, only, consulting detective. The Spanish fellow agreed and moved out shortly after.
A violin cacophony at three in the morning is often necessary for releasing emotion and stimulating thinking. A guitar melody at two in the afternoon is simply unacceptable. Good-bye number four.
Lestrade probably couldn't be called a proper flatmate. He was more of a watchdog/babysitter/whothehellknowswhat. He had walked into the flat with a baffling case involving a florist and a bee keeper and an unidentified poisoner. He had hoped to find an answer, or at least a direction, from the smug amateur that kept turning up at his crime scenes. Instead what he found was a shaking, sweating, and frightened young man trying very hard to control the body and mind the drugs had somehow found a way to steal from him. He stayed for three weeks, all the leave he had coming to him. He was never quite sure why he did and he never once regretted the decision. All the same, he was very happy when his wife called and insisted he come home.
Then came John. Right from the start the ex-army doctor was different. He stood up to Mycroft, called him brilliant, told him when he was being "a bit not good", and ran all over London just for the thrill of it. He complained about heads in the refrigerator and ears in jam jars, but he found some way to put up with them. He was kidnapped, more than once, concussed, shot at, strapped to a bomb and still he stayed. He was funny, kind, and quiet and most people made the mistake of thinking he was just another in a series of soon to be forgotten people that flitted in and out of the life of larger than life detective. Sherlock knew better. This man was a mystery, a constant question to be answered, the most interesting ordinary person he had ever met. Though he would find it very hard to admit, and even harder to express, he was very happy that this man, this John, had come along, and even happier that he had stayed.