Interesting thread. What music from 2018 could you send back to 1998 that first would blow them away and second might they not even recognise as music?
Trap, GQom. But also UK Dubstep from 2006-09. It's clearly inspired by the 'nuum and trip hop but it would be even more disturbing for them then it was for us.
There's a couple of James Blake tracks that would have really confused them as well.
Inspired by Mark Fisher (RIP) and Simon Reynolds.