They have many awesome songs, IMO. I like the combo of instruments with the soprano female vocals, unless they do a male-female duet as they did in The Pharaoh Sails to Orion and a couple others, which is also cool.
Floor has a wonderfully versatile vocal range and I find her habit of smiling when she's singing to be contagious. Anette sounds good on the songs that were written for her voice; I think she was better suited to singing for Disney than Nightwish though. I prefer more of the older stuff with Tarja - the notes she hits makes my spine tingle - so those songs dominate my favorites. Speaking of which, here are a few of them...
Dark Chest of Wonders
Stargazers
The Pharaoh Sails to Orion
Ghost Love Score
Endless Forms Most Beautiful
Moondance
Last Ride of the Day