DPP turned me into a fan of the band, it's a masterpiece, and Oceanborn is another amazing album, but the rest of Nightwish's repertoire is wildly inconsistent for my tastes and while they have a few awesome songs here and there, they have a lot of material I simply dislike. If I ranked them...
1. Dark Passion Play - flawless
2. Oceanborn - nearly flawless
[big gap]
3. Imaginaerum - very good
4. Endless Forms Most Beautiful - good
5. Once - half good, half poor
6. Century Child - half good, half poor
[big gap]
7. Wishmaster - very poor
8. Angels Fall First - very poor