My favorite places were La Cepa, La Viña, and Bar Txukun, all cheap and casual.
Lanziego was a step up and had some really nice old Riojas.
http://www.lanziego.com/es/carta-restaurante-donostia-san-sebastian.html
Next time I’m there I’ll reserve at Rekondo. No idea about the food but the wine list is insane.
Etxebarri is worth the trip.