i love you so much for this question this is the most serious question i've ever been asked and naturally i am gonna cheat and pick a whole bunch of songs :D
if we're going off just like my personal favorite sleep token songs i think Calcutta is a very txt-esque one and would suit them very well but when we talk about unlikely covers that i want deep in my heart anyway i'm about to pick a solo for each of them:
Soobin: Mine
Yeonjun: The Summoning
Beomgyu: Vore (realistically i have to acknowledge Rain suits him best at least from TMBTE but good god bg vore cover would fundamentally change me as a person)
Taehyun: Telomeres OR Past Self
Kai: The Offering (i have a vision and if you don't see it you simply have to trust me okay)

okay i'll stop there but this was very fun thank you for asking 🤎