San Miguel de Áralar (11,2km, 8,1%) is in Nafarroa, so it depends on whether you're restricting yourself to the Basque Country as in the Spanish entity or the lands claimed as Basque as a whole including Nafarroa and the French Basque country.
If you include the French Basque country then Errozate (10,1km, 9,6%) is probably toughest, but there's also Elhursaro/Arnostegui. In Navarre you also have the Higo de Monréal (8,8km, 9,5%), and some others.
If we're saying strictly País Vasco, then the main one is the Puerto de Urkiola (6,1km, 9,2%, though you can carry on a short while to the Santuario if you want). There's Pagolar (6,2km, 9,6%), Bikotx-Gane (5,1km, 8,3%), Larreder (7,2km, 8,2%, can easily be linked to Lamindao), Collado Elosua (9,7km, 5,6% from one side, 7,3km, 7,7% from the other), and one that I think should be used as a finish more often (i.e. for once), Sollube (hardest side 10,0km, 6.8%) but Urkiola is the main one that combines the toughness with being viable as a finish (Pagolar, for example, isn't). There's plenty of medium-length but less steep climbs, like Azurki, Asturiana, Urraki, Arantzazu and so on.
When it comes to shorter climbs, there's quite a few; this is where the Basque country excels. Aia is the most famous and among the hardest (1,4km, 14,4%), but there's also the Santuario de Virgén de Oro that the Euskal Bizikleta used to use, Asterrika (hardest side 2,3km, 9,6%), Kanpazar (3,0km, 9%), Lamindao (2,4km, 10%) and this utter beast: