Thank you for your question. The best cruise line for any itinerary will really depend on the individuals traveling and what preferences they may have. With that being said, it would be best to focus on cruise lines that sail more than just a few Northern Europe cruises per year. The top mainstream choices would be Holland America Line and Princess Cruises and the top premium/luxury choices would be Cunard Line & Oceania Cruises. Cruises to Northern Europe are seasonally offered from April through October, but some lines offer sailings at other times of the year. As for the best offers, Holland America and Princess Cruises have some great promotions currently that include many perks like drink packages, specialty dining and onboard credits.
Note: The links included above will take you directly to the cruise lines pages highlighting their Northern Europe cruises.