Coming from an RP-server, I've had occasion to use most of the built-in game emotes from time to time. But most of the time, they're too basic a tool for what I'm trying to convey. For example, /kiss doesn't tell you what kind of kiss it is - is it a peck on the cheek, an insincere air-kiss or a passionate snog.
That's why my favourite emote is ... /emote itself. With that, I can communicate greater nuances of meaning than a basic emote. Sven smiles coldly says something completely different to Sven smiles warmly or Sven smiles through the tears.