So after I've had Prism on reapet since yesterday, I'd rate the songs like this

Roar - Great lead single
9/10
Legendary Lovers - The chorus kinda reminds me of Counting Stars..definitely single material
9/10
Birthday - That song is soo overrated in the fan base...I mean it's cute but that's all
6/10
Walking On Air - Loved this song since the first listen..should stay an album track though
8/10
Unconditionally - Best choice for the second single..makes me emotional every listen
10/10
Dark Horse - This song is such a grower..I didn't really like DH at first but by now it's one of my fave Katy songs
10/10
This Is How We Do - Very funny bop
8/10
International Smile - Probably the blandest song on Prism...very uninspired and an awful filler
3/10
Ghost - Great mid-tempo track..really love the chorus
9/10
Love Me - Another great mid-tempo track
9/10
This Moment - The intro is life, though the lyrics are cringe-worthy here and there
8/10
Double Rainbow - Probably the stand-out track of the album..the lyrics and the chorus are everything
10/10
By The Grace Of God - Great ballad
8/10
Spiritual - A decent track, but also very overrated on atrl
7/10
It Takes Two - Amazing song that would deserve a place on the standard edition..the pre-chorus slays hard
10/10
Choose Your Battles - Another great bonus track which could have easily replaced Birthday or IS
8/10
Singles should go like this:
Roar
Unconditionally
Legendary Lovers
Dark Horse
Ghost / Double Rainbow