As always it depends on different things. If you are going with a caravan, or a bigger car, you shoud use the "Europaveier", green E-marks on the maps. (E-39 from Stavanger/Kristiansand to Bergen, and f.eks E-16 in return to Oslo. (If you start from Oslo, E-134 Haukelifjell will be a good alternative)
But if you have time, and a smaller car, this is my best advise:
If you comes from Denmark with ferry (Hirtshals) you can choose destination Kristiansand, (longest drive) Stavanger og Bergen (http://fjordline.com). Mark that there are two ferries between Stavanger and Bergen, some NOK 150 for the car and driver each ferry, and an amount also for the passenger. Next summer the most expensive toll on this road is finnished.
From Bergen you have different alternatives, depending on what you like to see. 1) E39, till you have crossed the Sognefjord, 2) even a trip up to Fjærland (a "booktown") and hotell Mundheim, 3) E 16 to Voss and Lærdalstunellen (25 km long tunel). My point is that you end up in Sogndal, where RV 55 starts, and it will take you over the mountains to Gudbrandsdalen, which you follow to Lillehammer, and then back to Oslo. You can hardly travel on the west coast of Norway without tollroads and ferries that you also have to pay.
The very old Walaker hotel (Wallaker.com) is a nice, but expensive place to stay, and the willage of Lom is one of the places in Norway with less rain. Here is different nise hotels and an stone owen bakery.
From the road between you have seen Galdøpiggen, the higest montain in Norway and lot of Jotun heimen as well. But again: This is a road for å small car and lots of time.
Well, I realize that this was more a mountin advice, than a fjord advice, but when you go from Bergen to Oslo, you are likely to cross the mountains.
Have a nice trip.