What are the must haves?
That's a tough question to answer, but I will attempt to explain why. For me, the earliest albums don't quite do it. It is very early in the "operatic metal" genre, and I don't think they pull it off well. I think they REALLY come into their own and find their sound on the last two albums with Tarja. From that era, I would recommend
Century Child and
Once, as well as the End of an Era live set (CD/DVD). To me, they never equaled this pairing of albums. The only thing is, Tarja is a phenomenal singer, but her vocals aren't for everyone, and it wouldn't surprise me if you didn't like her.
For the next era with Anette as singer, there are two albums.
Dark Passion Play has some good moments. But I think it falls short for a few reasons. One is that the music was written before they brought Anette in, and the songs aren't really written to her strengths at all. Also, the opener,
The Poet and the Pendulum, just gets the album off on the wrong foot for me. It is a huge epic, but is so self-indulgent as to just be eyeroll inducing for me. Couple that with a bunch of the tracks being Tuomas rants about Tarja, and the whole album just seems silly and pointless to me. The next album,
Imaginaerum, was written with Anette's voice in mind, and is fantastic.
For the Floor era, although she is easily the best singer Nightwish has ever had, and I love her being in the band, they have only done one studio album with her, and I don't like it. So while I feel that anything with her on it should get the nod, I can't recommend anything from the Floor era...yet.
So my recommendations boil down to:
-Century Child
-Once
-End of an Era (live)
-Imaginaerum