I figure the best thing to do is to explain which ones work and which ones didn't. It ultimately depends on what type of personality you're playing as. So I'll use my Shepards as examples. This may be considered long, so you've been warned.
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First up First up will be my female Shepard, Alaina (pronounced the same way as Elena). She is a near exclusive Paragon (always polite, always respectful, will not kill unless absolutely necessary).
Garrus:
I can NOT take this seriously. I mean it literally comes out of nowhere. There is no indication in either game that he has the slightest romantic interest in her. This was just a bone that they threw out to please all of the Garrus fangirls that popped up. I mean really look at this. One minute they're having a general conversation about the upcoming battle, then suddenly you basically have to choose to make Shepard whore herself out to him and he doesn't even believe it himself. Then he suddenly remembers he's a guy and says, "Okay, sure!" That's not to say that he isn't a great character. The loyalty mission of trying to pull him out of his descent into a cold blooded killer is one of my favorite sequences in the whole game. I just think throwing a romance on top of that was completely unnecessary. I mean, they've already developed a close friendship from both games. Why ruin that?
Jacob:
He would be my very close second choice. He's the first one (of the new characters) she meets, so she's known him the longest. And she does put out for him too, but at least there's no history between them to screw up. It's more like she's testing the waters to see if he's a viable possibility (It HAS been two years). Plus if you ultimately choose not to go through with it, there is a great scene when she is talking with Jacob about his father and how everyone she's known has changed and that is the perfect time to end it without anyone being hurt. Plus he's the one character that removes the most clothing in the love making scenes.
Not that I fantasize about that or anything...
Thane:
He is her first choice because he has the perfectly tragic setup. He's a widower, he feels his wife's death is his fault so he's completely racked with guilt. He's trying to reconnect with the son he felt he abandoned. He's dying of a terminal illness and wants to right any wrongs with the time he has left. So wouldn't you think that he deserves to be happy in his last days. Look at her reactios, she genuinely cares for him and wants to be there for him until the end. It's beautiful when you think about it.
Next up are the choices for my male Renegade, Nathan. (Always cocky, rude to time wasters, kills anyone he feels deserves it, but ultimately does the right thing.)
Tali:
Unlike Garrus, this at least has a believeable setup. She's completely enamored in him from the moment she sees him alive again. And she demonstrates that she is perfectly willing to disregard even her own beliefs if Shepard asked her to. Plus the fact that she's the one that inadvertantly blurts out that she secretly had feelings for him since the first game but never acted on them. She thinks of herself as the mundane colleague that probably wouldn't register in his mind as a love interest. That makes much more sense than her feelings coming out of nowhere. But here's the thing, the story works best when she doesn't ultimately act upon them. She shows how much she truely cares about him by not burdening him with this complex relationship where they can't physically touch each other without worrying whether or not she might die. Just each of them letting the other know that they could if that wasn't hanging over their heads is good enough for them.
That leaves us with Miranda and Jack:
Honestly, it's a toss up between the two. Either one could work, in my opinion. They both have these hard, unobtainable personas that they try to project. And both of those personas start to break as you talk to them. I personally love the way he keeps bugging Jack and the way she doesn't know how to handle it. I lean a little towards Miranda because of how cold she is at the beginning and how she warms up by the end. But as I said, either way works for me.
) very attractive. Garrus, because he was a good buddy in my previous playthroughs, and I had heard about how awkward the whole, uh, "encounter" was.