Nightwish with Tarja is one of my favorite bands of all time. Oceanborn is one of my favorite albums, probably my top 5. Nightwish with Tarja is hard to put your finger on as far as genre. Maybe say a mix of traditional and synth heavy power metal fronted by a legitimate soprano.
They have always been very polarizing in the metal world. I reside on the edge of favor, personally. Though I don't acknowledge Nightwish after she was fired from the band. To my ears, she was the most important piece that made them tick, regardless of whether she was a complete diva bitch to the rest of the band or not.