Garrus, and here's why -

Whether you romance him or not, Garrus has some of the biggest character development throughout the entire series if he's interacted with in a certain way. When you meet him, he's impatient and very much in an anti-authoritative phase after his experience with C-Sec, and has a blatant "take action first, ask questions" later attitude that had evidently gotten him on thin ice before. Once he meets Shepard, and if he's edged to more of a "paragon" path, he grows up a bit and starts taking his actions a bit more seriously. In ME1 this is evident if Shepard convinces him to attempt to take Dr. Saleon into questioning instead of killing him right away. In ME2, it happens if Shepard convinces him not to kill Sidonis.

By the time he comes around in ME3, he is very respected among the turian military (a significant step up from his stint in C-Sec) and it's obvious that his attitude has mellowed considerably and that his actions are much more calculated and much less reliant on reactive instinct. He chooses to pull the turian fleet back when he realizes the Reapers are too much for them. Arguably, the Garrus of ME1 wouldn't have done the same. Through Shepard's influence, he becomes much more methodic, a lot less trigger-happy, and more empathetic.

Now, the reason that I love adding romance into this is because I find something very, very fulfilling about the idea of Garrus being led to that point by what amounted to being, by his own admission in ME2, "the only friend he's got left in this screwed up galaxy," and realizing over time that she's come to mean more to him than that. When Shepard died in the beginning of ME2, Garrus lost the only real friend he had, and when she was gone, he fell back into his pattern of reckless behavior by donning the Archangel persona. According to Mass Effect: Homeworlds, he had resigned himself to death when he found himself cornered by the Omega merc groups, and it was only when he saw that Shepard was alive again that he found the motivation to keep fighting. In a potential romantic context, that is just too sweet.

Not to mention that Garrus is the only one that has stood by Shepard from the beginning with no questions asked (also Wrex, but he was essentially leading Tuchanka by the time ME2 came around, so I can't really fault him for that). Kaidan and Ashley chew Shepard out on Horizon and accuse her of losing her way and allying with the enemy. While Liara did save Shepard's life, her priorities lied with her vendetta against the Shadow Broker. Even Tali had a hard time coming around knowing that Shepard was working with Cerberus. But Garrus? No questions asked. He saw she was alive and didn't give two shits that Cerberus was involved. His friend was alive, and that's all that mattered to him.

Having them fall in love in that context makes it one of my favorite character arcs in any game, period.