On male Shep, i tried to go for Jack, but I accidently broke it off D:, so I settled for Tali to get the achievement. I thought Jack was really awesome, what with all the crazy biotic powers, and I'm a sucker for those tats and DEM LIPS.
Tali was ok, a bit annoying, but I always thought of her more like an awesome cousin or something, someione who is really cool but is still somewhat in the sidelines. (Guess thats why I always recruit her last, except for Legion of course.)
And on my current femshep, I am romancing Thane, since he seemed like a real nice guy, and not to mention awesomely religious-y, without sounding like a moron.
I did have a minor conflict where I almost got with Garrus. Another time, I guess.
....y'know, re-reading this makes me think that I'm attracted to the whole "likes-to-kill-people-but-really-has-a-lot-of-commitment-issues/Kills-people-for-money-and-is-dead-inside-but-finds-love-close-to-death". I guess I am.