I know, I know, the correct answer is "all of them" but we don't want to kill the band, let's be generously conservative and say 6.
The Kinslayer- Not only because its a great uptemo riffy number, but it's social relevance is staggering. It's more relevant now than it was in 2000 (and Tuomas was the first metal band to write a song about a school shooting, and it was blazing speed metal! A testament to his ingenuity)
Beauty of the Beast- So underrated. The lyric "all of my songs can only be composed of the greatest of pains" should be sung live by any Nightwish singer. They've also done every other epic number with Floor so I don't see why this one can't come out.
Dead Boy's Poem- While it's not in my personal top 20 this song was a gold standard for fans for many years. I also liked how Emppu turned it into his own solo section during the Anette years. If it's good enough for Annette, it's good enough for Floor.
Tutankhamen- Angels Fall First is not a bad album. It's just a little rough around the edges. There's plenty of worthy songs to pick from it, but we've all heard Tuomas jam the lick to this during The Siren a couple times. He did the same thing with Stargazers with Planet Hell, so I think he's itching to bring it out (I hope). It's got a great Rainbow Rising vibe to it. And that's what made Nightwish great. They were Ronnie-era Rainbow worship with a female singer!
10th Man Down- AKJdnkajsdnjkasndaskdsdfdnkdasmgakfiefowaekfpowefm So fucking good! So 80s metal (the good, Dio kind)! Floor would be majestic on this chorus!
Passion & The Opera- Don't act like Floor can't pull it off. (also, THAT RIFF!)
Bonus: Pharoah Sails To Orion- Troy could do the flute solo on his pipes.
Addendum: It's come to my attention that not everyone is a setlist nerd like me and knows what has been played, so I'm going to list what has been here. Also "old" counts as everything before EFMB.
Old songs already done with Floor:
She Is My Sin
Over The Hills & Far Away
Bless The Child
Dead To The World
Dark Chest of Wonders
Wish I Had An Angel
Ghost Love Score
Higher Than Hope
Poet & The Pendulum
7 Days To The Wolves
I Want My Tears Back
The Crow, The Owl and The Dove
Slow, Love Slow
Last Ride of The Day
Song of Myself