It's a bit basic but it's probably Doctor. It's just so good. Descending to land of wind and shade as you hear Doctor for the first time is an experience unlike anything else in the comic.

Some other picks are Do you remem8er me, Descend, unite synchronisation, green ghost, umbral ultimatum, homestuck anthem, honestly I could go on. Four kids (and a game they played together) for the beyond canon shout out.