So who is the "Betrayer"of Daenerys?

Daenerys has done nothing but take L's since she's arrived in Westeros. Every move she's made so far has been anticipated perfectly with large portions of her fleet destroyed, key allies taken and the Unsullied caught in a trap set for them at Casterly Rock. Either this is the laziest writing ever for the sake of creating drama and suspense because the plot needs Euron to feel like a legitimate "big bad" threat or (better be the case) one of Daenery's key allies is actually a mole who has been leaking information to Cersei. The likely suspect seems to be Varys considering Daenery's "you better not betray me" speech in the previous episode but I can't conceive why he would possibly want to see Cersei in power so I wager it is most likely someone else.
It can't possibly be Ellaria sand given her blind hatred of anything Lannister or Yara considering the position they are both in. Lady Tyrell has more reason to hate Cersei than anyone and she's now dead so that crosses her out. So who? Missande? Some misguided attempt to protect Grey Worm? Varys? History of betraying those he served and warned not to do it again.

Nobody really seems to make any sense at all besides "maybe" a scorned Daario (who has betrayed allies before) but I don't know how they'd explain how he had access/knowledge of Daenery's game plan.