You know, I love Tali, and I love her romance. But holy crap. That singing, I was literally cringing while watching it. I was trying so hard to like it, but I couldn't. I love every other aspect of her romance that I've seen...except that...
Liara, being the Dev Pet is of course done basically perfectly, I cannot make a single complaint about all of Liara's romance. it just flows so naturally through the 3 games. I always had the impression that even if your with someone else in the series, she is still in love with you regardless. It just works effortlessly. Using her as inspiration is a great place to start for any RPG, imo.
Now, I admit being denied my lovely Aveline Vallen left me totally devastated for a long while. Even OOCly I was obsessed with her, I wanted her so bad I wrote a steamy fan fiction about her before the game even came out. However, Cassandra, for me at least was without question the best DA romance I've gotten. She clicked with me like nothing else. I myself am not very PC, I am blunt and difficult, but inside I myself am more. I understood her so well it was like it was made for me. Even her weird morbid pillow talk makes sense for her. As she said, she does not swoon.