I figured it might be "Radioactive" since Harmonix already had their hands on the stems from Disney Fantasia.
I'm too lazy to do any matching up myself, but I wonder how much overlap there is between RB and Fantasia at this point.
from the hip, songs in RB bolded
Dvořák – “Symphony No. 9 from the New World”
Vivaldi – “The Four Seasons: Winter, 1st Movement”
Avicii– “Levels”
J.S. Bach – “Toccata and Fugue in D Minor”
Bruno Mars – “Locked Out of Heaven”
Cee Lo Green – “Forget You”
David Bowie – “Ziggy Stardust”
Depeche Mode – “Enjoy the Silence”
Drake – “Take Care” (ft. Rihanna)
Elton John – “Rocket Man”
The Flaming Lips “Yoshimi Battles The Pink Robots, Pt. 1”
Fun. – “Some Nights”
Gorillaz – “Feel Good Inc.”
Imagine Dragons “Radioactive”
Inon Zur – “Scout’s Theme” (ft. Lindsey Stirling)
Jimi Hendrix “Fire”
Kimbra – “Settle Down”
Lady Gaga “Applause”
Liszt “Hungarian Rhapsody No. 2”
Lorde – “Royals”
M.I.A. – “Galang”
Missy Elliot “Get Ur Freak On”
Mussorgsky – “Night on Bald Mountain” (RBN)
New Order – “Blue Monday”
Nicki Minaj – “Super Bass”
Peter Gabriel – “In Your Eyes”
Police – “Message in a Bottle”
Queen – “Bohemian Rhapsody”
Tchaikovsky – “The Nutcracker Medley”
The Who – “The Real Me”
The White Stripes – “Seven Nation Army”
Mozart – “Eine Kleine Nachtmusik”
DLC:
Coldplay - Paradise
Soundgarden - Spoonman
Skrillex - Try It Out
Ellie Goulding - Burn
The Cure - Just Like Heaven
Maroon 5 - Lucky Strike
Talking Heads - Burning Down The House
The Band Perry - DONE
Zedd ftt Hayley Williams - Stay the Night
OneRepublic - Counting Stars
Lady Gaga - The Edge of Glory
Demi Lovato - Let It Go
REM - End of of the World As We Know It
Avicii - Lay Me Down
Avicii - You Make Me
Justin Bieber - As Long as You Love Me
Closer - Ne-Yo
Flo Rida - I Cry