There are so many candidates, but here are a few obvious ones (wether big guns are a main focus or not...) - especially considering the surrounding nature:
* MKB Engeløy (Batterie Dietl)
* MKB Kroodden (Batterie Vara)
* MKB Örlandet (Austrått fortress)
* MKB Fjell
* Herdla
* Torpedobatterie Nordlandet (+ Julholmen and Ledangsholm, seen from the sea)
* HKB Utvorden-Nord
* HKB Elgsnes and HKB Grøtavær
* The german tank shooting-range at Trandum (although not a coastal defence installation, it's still a spectacular site I think).
* All (?) batteries in Lofoten and Finnmark, although most places you'll get more nature and less remains of concrete and steel...
* +++
Most sites are well-documented in online forums like "Kystfort.com", as well as in books (for instance, the first two volumes of J-B. Wahl's trilogy of the atlantic wall in Norway should provide a good reference with it's numerous photos). I suggest browsing through the topics in here and in available books to make an educated decision
