Most interesting: Quarians or Asari. This might be a little biased toward Shepard's own interests, (girly crush on one, dating the other) but both species are pretty interesting. Quarians are neat because while Shepard may have been lucky to be void-born, the entire Quarian race has been living that way for millenias. she doesn't really know, but to have entire colonies living on a single ship is bound to make rituals and such quite unique.

Asari are just as neat not only because of their extended life-span, but also being entirely female. Shepard still doesn't entirely get how Asari procreation works, but its definitely not the typical biological way. She's also a little blown over just how long Asari live. I mean, 1000 years? That's just crazy and she has to wonder what they do every day without being bored all the time. They're appealing to the eyes too, but that's kinda obvious.

Least interesting: Batarians. Fuck them.