The Poet And The Pendulum and Ghost Love Score are two songs that stand out the most who always covers my body in goosebumps and sends shivers down my spine. But also The Siren, 7 Days To The Wolves, Slaying The Dreamer, Ever Dream, Wishmaster... well, I can be here all night listing songs that are just frakken awesome. XD
One thing I love about Nightwish is they have a good array of songs that are awesomely upbeat, fun to sing along too and can be slow and relaxing. ^_^