Loki, and here's why....

- there's been a lot of mentions of Asgard and Asgardians on the show so far.

- causing chaos from afar is totally Loki's style.

- his seeming obsession with Coulson. he personally killed him and it's bugging the crap out of him as to why he doesn't know how he's still a alive.

- he's got beef with S.H.I.E.L.D. for assembling the Avengers together to stop him.

- i think he was involved in sending Sif to retrieve Lorelei. Sif says that Odin ordered her to bring her back alive. remember at the end of Thor 2, Loki is impersonating Odin.

- Coulson said that there's no one with such a power as clairvoyance in the index (no mutants) but we know that Asgardians have access to such a power (or at least technology) similar to Heimdall.