Given the history between the two, the Tali romance makes the most sense in the storyline. However, as we all are gamers thus will take any excuse to get with the hottest girl in the game, Miranda is the obvious option. Zero (I REFUSE to call her "Jack") is the romance you choose when you're playing the renegade playthrough.
For the female Sheperds, Jacob is the Kaiden of ME2 (thus no one really notices nor cares about him once you get Garrus in your squad). Garrus (while a great sidekick and teemmate) is a HORRIBLE choice for a romance not just because of his lame final romance scene but because...that's just wierd, so really Thane is the #1 choice of that group by default.
So really, Miranda for Male Shepherd, Thane for Female Shepherd, Tali for Story interested Shepherd, Zero for Renegade Shepherd and everyone else....well everyone else kinda sucks....yes even Kelly. Her's doesn't even make sense.
Seriously, what idiot picked Morinth?
Edit: F@#k Ashley. F@#k Liara too. You must pick a new romance or FAIL.