Definitely Mako! I was so bummed that I couldn't romance her as a female BH, so I ended up rolling another BH(this time powertech for a different play style. Plus Mako heals very well, so you'll be playing with her all the way to 50, especially if you go tank spec.)
It's hard to put into words just how great Mako is as a companion or her character roll in general. The only way to really put it is, go roll a male BH!
Also, a major plus(for me at least) is that Mako's voice actress is none other than the very beautiful Lacey Chabert!