The Blood Pack are unreliable and seem largely ineffectual, the Eclipse are too bloodthirsty, but the Blue Sun just tend to do shady things. I'd have an easier time joining the Blue Suns than the Eclipse or Blood Pack, I think.
Still, the Eclipse seem on average the most badass, the most apt to work as a unit, and in general the group with the most potential for awesome less-than-legal adventures in the ME universe. Were it not, of course, for the horrible things they do.
Oh yeah, and for the Asari, of course.