Welcome to the board, both of you.
I closed the other topic, since it is a duplicate, and since someone replied in this one, this will be the "official Nightwish recommendations" topic.
As a Nightwish fan for almost 10 years, I have lots of recommendations! Since you are new to the band, I should probably start off by telling you that the band had 2 female vocalists. The first 5 albums (Angels Fall First, Oceanborn, Wishmaster, Century Child, Once), as well as the EP Over the Hills and Far Away, features former lead vocalist Tarja Turunen, who was more of an operatic/classical sensibility. The band's last album, Dark Passion Play, and the album they are currently working on in the studio (Imaginarium) feature their current vocalist, Anette Olzon, who is more of a rock/pop-style of singer. Definitely two vastly different voices in this band, which has led to many debates among fans over who is "better" for the band (personally, I like them both and think both bring something different to the table so there is no use to argue over it since they are two different styles and neither one of them could provide that which the other brought to the group). The band has also had male voices throughout their career: keyboardist/songwriter Tuomas Holopainen provided vocals on their first album "Angels Fall First"; Tapio Wilska (a former bandmate of Tuomas' in another group) did vocals for Oceanborn and the song "10th Man Down" on their Over the Hills and Far Away EP. Tony Kakko from Sonata Arctica did some guest spots, one on their live album/DVD From Wishes to Eternity, and another for a remake of the Angels Fall First song "Astral Romance", which also appears on the Over the Hills EP. But the main male vocalist since 2002 has been Marco Hietala, who is also the bassist of Nightwish (as well as bassist/vocalist for the Finnish band Tarot as well as countless other side-projects that even I cannot keep track of!). For one song only, there was even a 3rd female vocalist when Indica frontwoman Jonsuu guested on the bonus track "Eramaan Viimeinen", which is better known on the Dark Passion Play album as an instrumental track called "Last of the Wilds".
Since it appears you are familiar mostly with the Tarja-era stuff at this time, I would agree with the post above that Once is a good recommendation, as it is their last album with Tarja and musically is a good indicator of the direction they have moved in since. But I would also recommend Dark Passion Play, which at this time is their most recent album (that is, until the new one comes out early next year), so it gives you an idea of how they sound now compared to their earlier albums.
Really though, you can't go wrong with any Nightwish album. Since you seem to be familiar with mostly Wishmaster-era stuff, a natural progression from there would be to check out Oceanborn, since Wishmaster is considered by Tuomas (the main songwriter of the band) to be "a sequel to Oceanborn". Oceanborn is considered by many Nightwish fans to be their finest work, and while I can't disagree that it's a fantastic album, I am partial to Wishmaster myself. Many fans are also fond of Century Child, which is a great album as well. Their first album, Angels Fall First, is also a good album but not terribly indicative of what their musical direction would become.
There are also live albums and DVDs you can check out; their most recent live album is Made in Hong Kong (And Other Various Places), but those are all DPP tracks done live, so it doesn't really cover a wide array of their catalog like past live albums do.
Hopefully this helps you a little on your Nightwish journey. You can also click on the second banner in my sig to access the OSA site, where we have tons more information about the band and where you can download a special live track that is exclusive only on our website!
Welcome aboard!
