I'd call Wishmaster the peak of the early era, full of youthful exuberance and enthusiasm. The debut was, well, a debut, Oceanborn I recognize it's good but I personally like way more Wishmaster, the band was having a lot of fun there, they were young and hungry.
Then they went in pairs I guess, adopting a style, or a singer, and perfectioning it. Bless the Child was a switch to a more melanchonic atmosphere, and then came the better Once. Then came Annette, with Dark Passion Play a stepping stone for Imaginaerum which is still my favorite Nightwish album. Then came Floor, and again I feel that EFMB was the "safe album with the new singer" and Human || Nature was them getting to know better the singer and going wild with it.