Best central location, Guggenheim or old town?
We stayed in the old town after our recent RSE Basque Country Tour, and loved it.
Our hotel was the Petit Palace Arana, just on the edge of the old town, a wonderful location. We liked the hotel - great staff, good breakfast.
As a Bilbao local...it´s a very compact and walkable city, you get on foot practically everywhere. Both locations are good, it´s just a very nice 15-20min walk distance one from the other.
We stayed at the Petite Palace Arana as well, in Old Town. I think Old Town probably offers more in the way of an evening walk and activities, which I like close to my hotel.