For my mainshep, I would say that her best friends are Thane, Jack and Tali. Garrus is her lover of course. Thane is this deadly sniper who she finds very... intriguing to use his words.
She had a similar background to Jack because she was on Akuze, Cerberus basically tortured her and turned her life into hell too. Of course, she would never say that to Jack, because that might belittle Jack's situation, but she really empathises with the crazy biotic.
Tali is like a little sister who she feels obligated to protect. They get along well and can talk really easily to each other, having known each other for a while. Shepard is also really proud of how Tali has developed in ME2 (and the same with Garrus.) Wrex is a bro too. Nuff said.
And, because you all seem to view Garrus as a friend, and you little minx's can't get your hands off Thane

(you know who you are!!) Then I shall tell you why I prefer Garrus, because I understand you may find it baffling. Hehe.
Garrus has a relationship development from ME1-ME2. They are now equals. They have this massive trust in each other, and they know that the other has their back no matter what. This is something unique to the relationship between Garrus and Shepard. They have both been to hell and back and have learned from it. And one of the best things about Garrus is how he deals with the news that Shepard is working with Cerberus. Unlike the virmire survivor, he trusts Shepard that much that Cerberus doesn't matter. Even Tali doesn't like it. But Garrus sees it as an opportunity to watch Shepard's back even more closely.
Also, during Garrus' loyalty mission, if you pick the paragon path with Sidonis, Shepard confidently tells him, 'I am the only thing standing between you and a hole in head.' She is totally confident that Garrus wouldn't risk a shot with her there, even for the sake of avenging the deaths of his team. And Garrus is a pretty vengeful guy.
His romance reveals a whole different side to our snipering turian cop, he has tenderness, and a bit of vulnerability. It's also a relationship that isn't shallow (like Jacob's.) They are both willing to risk a lot just by being together. There are cultural differences as well as physical differences. Their personalities are similar, and they complement each other in the romance option. If Garrus opens his feelings a little, Shepard easily returns it, and vice versa. If Garrus says something awkward, Shepard just laughs it off. They fit so well together. But the main thing for me, is the fact that Garrus has been through so much, and he is still able to make jokes, move on, become stronger and learn from the experience.
So there you have it my fellow drellickers. The mtstery has been revealed