I don't think it's Lenalee, Allen, Cross, or any of the generals, because in chapter 158 the earl states that the heart is camouflaging itself and creating a few fake hearts to throw them off.
So it's unlikely for any of them to posses it, but it IS still possible (the heart could just be using reverse-reverse psychology).
But what interests me is that the Earl is referring to the heart as if it was a person or retained its own will and thought.
Also, I think the heart is controlled by Miranda, Kanda, Krory, or the mysterious thief G.
Miranda, because she's the ONLY non-combatant innocence that also controls time.
Kanda, because of his tattoo, and because he's the first and only (as of now) exorcist that has killed a Noah.
Krory, because he obtained his
parasitic-type in an unsual way (he wasn't born with it). And
The Mysterious Thief G because he has a strange and unknown ability that makes you nearly die laughing.