Liara, With the LOTSB dlc for ME:2 it's so much more meaningful than any other relationship option in either game (What with the back story and all).
Kaidan is just so, blah. His one character aspect is "boo hoo, My implants hurt" and thats why I prefer nuking him over Ash.
edit: Or you could not romance anyone and pretend your laying the groundwork for Garrus in ME:2 because lets be frank; Garrus!