Liara or Tali, mostly for the sake of consistence across all three games.
I prefer Tali because Liara sounds like an extremely naive stoner lol. Plus she looks CREEPY. (Fully human face stapled onto an otherwise alien head...what?!). The asari look less "human face" so it just makes her stand out even more. Wasn't she directly modeled after someone like other crew members?
That said I think Tali is the best for various reasons
a) Him romancing the "outcast" shows depth for shepard
Tali grows with shepard throughout the trilogy
c) Tali is present as a squadmate for the entirely trilogy
d) I like the consistent dynamic of Soldier Shepard, Leader, Sniper Garrus, Lancer and best friend, Engineer Tali, Support and Love Interest.
Plus with cases like Jack or Miranda, it feels more like Shepard is their psychologist giving therapy sessions, rather than a romantic interest most of the time. And others like Kelly Chambers are just a "fling", then ME3 only interests like Kaiden and Steve are out of nowhere and too "last minute" for me to really care about.