I loved to stay at Olaia&Marco, the apartment is beautiful, very clean and my room looked exactly as in the photos. Marco was very nice and helpful and even changed the check in hours for my confort.
The apartment is in a beautiful building, very central, 5 min walking to Guggenheim and to Casco Viejo, as for the airport bus. Many pinchos restaurants and stores around.
I will definitely come back to Bilbao and stay here again.