We had the same question last fall when planning our Scandinavian trip for this summer. My husband and I have done NIN plus Sognefjord in 2000 but the kids haven't seen the fjords.
We were considering Geirangerfjord or going to the Lofoten. In the end, the uniqueness of Lofoten and people's raves that it really is stunning won out and we are headed there. We will fly Oslo to Harstad/Narvik Airport (EVE) and rent a car at the airport. It is then a 2 hour drive to Svolvaer where we will spend our first night. We like this better than flying to Bodo and taking the ferry, less chances for schedule delays/mishaps matching up flight times/ferries. Plus renting a car at EVE was cheaper/easier than Svolvaer of Moskenes (or Bodo having to ferry it across). You can also fly to Svolvaer or Leknes but flights were twice as expensive.