Best part of the Camino del Norte?

I would agree with 2 caveats. First he climb out off Irun (El Purgatorio) was for me the most demanding section of the entire Camino del Norte/Camino Primitivo/Camino fisterre that I walked in 2018, and its the first couple hours of the first day on the trail!
Second, the last day into Bilbao was a total waste, walking on paved roads thru industrial areas.
The section between Playa la Arena and Santander were my favorites on the del Norte. I made every effort to walk as much as possible along the coast and in retrospect it was well worth the effort.

Contrariwise I would strongly advocate for Irun to Bilbao. Close to the sea, scenic villages and some spectacular industrial ruins on the final day into Bilbao. A ruined fishing industry, collapsed steel manufacture, abandoned ship-building yards and a dead posh art gallery designed by an outsider and built out of imported materials. What’s not to like.
If you want pretty try Santander to the aptly named Villaviciosa