Hmm, I am going to buck the trend here. I think Dark Passion Play was a better album than the two which preceded it. Once has a several killer tracks, but quite a few that don't do much for me. Likewise with Century Child which blasts out of the gate only to lose steam as it goes on.
For me, DPP is just a more consistent album with only "Sahara" and "Whoever Brings the Night" not being particularly memorable.
I also think that my enjoyment of Nightwish has been dictated by Tuomas' style of songwriting and not so much whomever the singer happens to be. For sure, Tarja was cool, and when I first heard Nightwish, it was certainly her vocals that immediately caught my attention. However, that wouldn't have carried me through to the enjoyment, and even fufillment I get from their records, without some very good songwriting.
While Annette is not the powerhouse that Tarja is, I still enjoy her voice greatly, and it doesn't significantly change my overall enjoyment of Nightwish. I'm interested to hear their next album, and see if she imparts more of her own flavor into it. I'm actually glad they brought it someone who sounds different from Tarja, now that I think about it, because at least we didn't get stuck with a poor imitation of the real article (a la Tim Owens replacing Rob Halford in Judas Priest).
To me, Wishmaster is their best overall album, as I really like every song on it. DPP and Oceanborn are close for second place, but ultimately, DPP gets it, due the presence of several epic tracks, whereas Oceanborn had a more basic sound overall.