Don't have an extensive knowledge of Viking metal, but I can tell you it rules...and that Bathory is the fuckin' greatest of all
Best of all would be Bathory; epic, epic, and more epic...check out One Rode To Asa Bay...flat out best song ever. Video:
http://www.youtube.com/watch?v=hDpc-831GPs
Also, listen to Song To Hall Up High, Home Of Once Brave, Baptised In Fire And Ice, Father To Son, Bond Of Blood, and For All Those Who Died
Also, Enslaved is pretty good, more of a black metal thing.
If you like melodic death metal, check out Amon Amarth; you'll probably hear something off the new album, if you don't like it check out the older stuff cuz it's all good but it's a totally different style. Asator, Under The North Star, Runes To My Memory, An Ancient Sign of Coming Storm, Victorious March, Death In Fire, and of course The Dragon's Flight Across The Waves for starters...
If you dig viking metal it might be worth your while to check out some other folk-metal; Eluveitie, Skyclad, and Elvenking are all fuckin' awesome.