Staying at Heartlock's place was the best choice!
First of all the location is great, not too touristic but still central and with a very nice nightlife, chill but active. Then, the comforts offered by the (not-so-small) apartment make it feel more like an hotel than like a common Airbnb, with small attentions to spoil you a bit. Communication and check-in were very good (I even had the opportunity to check in a bit earlier than what wrote in the website since my flight was arriving early in the morning and had some suggestions about places in the neighborhood).
Overall, I'll happen to be again in Athens I'd definitely try to come back to Heartlock's and I'd definitely suggest the place to all of my friends.