My top few, in approximate order of how far my jaw dropped:
I was glad to see that a couple of people have Abyssal Lurker, who I think is probably greatest KSP pilot of all time. You should watch every single one of their (sadly small) catalog of videos.
Joint second place to @pa1983 and @Overfloater for some of the best early heavy-lift SSTOs. Back in the era of intake spamming and the pre-1.0 souposphere, @pa1983 created some of the first really large SSTOs, and @Overfloater further refined the genre, creating some of the most truly elegant and functional craft of period. I don't know if any of the former's videos survive, but check out MrOverFloater on YouTube.
@astrobond for creating the first Eve SSTO, and @Stratzenblitz75 for the first fully-reusable Eve mission.
I'd be remiss not to mention Scott Manley, who has given this community so much. I second @Piper's nomination of his Orange Efficiency mission as a particular highlight.
The second-greatest pilot of all time is probably @Cupcake...—even though they started out only just barely managing to build a rocket and have now gone completely insane.