OCEANBORN is their best album by far. Wishmaster is also pretty swell.
If you want to hear a band better than Nightwish with a Female vocalist get Aghora's "Formless". I just got it & it's an amazing work. I believe this album took 6 years to make. The vocalist is a lot different than Tarja, however, but they're much more Progressive, technical, & TALENTED.
I also adore Stream of Passion's only album "Embrace the Storm". It's much different that Nightwish though. Great guitar leads by Lori Linstruth & a really good chick vocalist. Therion's new album has some great chick vocals too, but you're best off with the Aghora album for now.
I would say Aghora are a unique band, but Nightwish just have their own sound. I've never heard a Nightwish clone myself. I'm guessing there are some out there, but they're probably pretty generic sounding.