Century Child is my favorite album because it has a lot more emotions in it than any of the other albums. Century Child has my favorite song of all time (Ever Dream) in it and out of my top 10 Nightwish songs, 3 of them are from Century Child. For the remaining 7 songs in the top 10, there are 2 songs each from Wishmaster, Once, and Oceanborn, 1 from Dark Passion Play, and none from Angels Fall First.
Since I see other people ranking their albums from most favorite to least favorite, I'll do the same thing.
1. Century Child
2. Wishmaster
3. Once
4. Oceanborn
5. Dark Passion Play
6. Angels Fall First
For a while, I thought Wishmaster and Once were equal for me, until I really decided to take a look at the individual songs for both albums. After looking into both albums in more detail, I have determined that Wishmaster would be ranked higher up on my list than Once.