For me it was Miranda first. Because she's hot and so is her accent. In the same file though, I also had an apparently meaningless one night stand with Jack, I guess because Miranda was busy. Shepherd is such a slut, s/he'll bone anything that moves.

In my second, paragon file, I'm going for Tali, simply because I haven't done her yet. Because that's what Shepherd's real mission is apparently. She's hot enough under the suit as there's some mystery behind it, but if one were to actually see what she looked like, it would probably be nasty. Quarians have similar amino acids to Turians - not sure if that translates into similar appearances/skin/form/whatever. Oh, and Kelly's fair game this file.

@Tiff
Go for Jacob. You won't be able to go back afterwards.