Okay well it really depends on the person. Someone who like rock music, I'd probably start them out with a fast-paced, heavier song. Someone who isn't really into rock I'd start them out with a slower, softer song. Plus someone who thinks anything remotely dark is emo would rather listen to a song like "Amaranth" instead of, say, "End of All Hope". But, here's a general list, in no particular order.
Reason: This was my very first Nightwish song, and it's still a favorite. It's just a great introductory song.
2. "Planet Hell"
Reason: The beginning of this song just raises goosebumps, and it just gets better from there.
3. "The Islander"
Reason: It's quite different from their normal stuff. It's more folkish and sad. Not to mention it's mostly Marco singing, which is rare .... usually it's a female singing.
4. "Ghost Love Score"
Reason: This may be one of my favorite NW songs. It flows so well and so beautifully, and Tarja's voice just breaks my heart in some parts.
5. "Slaying the Dreamer"
Reason: Always good for a rock or metal fan. The best part is halfway through.
Reason: This song just screams NIGHTWISH! to me. The music, the vocals, the lyrics ..... it all ties together.
7. "Sleeping Sun"
Reason: A sad-yet-soothing slow song .... it's lovely.
8. "Angels Fall First"
Reason: Gotta throw a song in from the album with the same name, because their music changed a whole lot after this. Great lyrics in this one.
9. "The Poet and The Pendulum"
Reason: I haven't given Anette fair coverage, and this is an epic song.
10. "The Siren"
Reason: Because I told you so. *wonders if anyone will get this far*
Also I'd like to toss out a few Within Temptation songs. Everyone listens to "What Have You Done" or "Frozen" and goes "Eh." but they've got way better stuff.
1. "Mother Earth" (and listening to 30 seconds does not count.) - I love this song, and Sharon sings it so well.
2. "Deceiver of Fools"
3. "See Who I Am"
4. "Hand of Sorrow"
5. "Running Up That Hill" (as much as I like Kate Bush, I prefer WT's cover)
And just to be silly, "Gothic Christmas". I swear, that song cracks me up every time.