Can't say I know too much about metal, but Nightwish was definitely a huge part of my middle/high school days. I am one of those people who definitely preferred Tarja, but honestly Annette was good too. A bit weak on Dark Passion Play, but it's not like she was bad. Her voice did not suit some of the songs, and I remember some people saying that she sounded too much like a pop star. Imaginaerium, however, was really wonderful. I'm not sure what changed, but her vocals and the music complemented each other much better. I have not listened to the latest album, though. It's been a while since I had a real interest in Nightwish.
As far as my favourite albums, well, it would be easier to say that I don't care for the first one much at all and Dark Passion Play, while some of the composition is quite nice, just has too much of a clash between Annette and everything else. For songs, I like "Deep Silent Complete", "The Crow, the Owl, and the Dove", "The Islander", and at the top probably is "The Siren".
Like I said, I don't listen to much in terms of metal. Other than Nightwish, it's mostly just Wolves in the Throne Room and Lacrimas Profundere. Wolves can be a bit too heavy and screamo for me, but "Diadem of 12 Stars" is pretty awesome, save for a dull 5 minute section in the middle. Lacrimas is generally good. I'm pretty on-board with German singers doing English lyrics. Sometimes there is a point where I'm not entirely sure what they're trying to describe, but great vocals.