:3 is that which transcends all other emotes, because it is open-ended and ambiguous. I probably use :3 more than I should, but it's incredibly easy to use when you just don't have anything, well, better to say. I only wish I could use it with people I see face to face everyday, and I often wonder how one could express :3 with one's body language or face. In any case, here's some reactionary examples of just how incredibly useful :3 is.

Pretty :3

Cute :3

Didn't see this one coming. :3

I'll pass, thank you. :3

Uh :3

.....:3