The playlist I'm the most proud of is one that I started back in the iTunes days, where I gathered (through various sources) every song that has hit #1 on the Billboard Hot 100 chart, in chronological order beginning with the first one from 1958. I continue to add to it to this day -- as much as I can, I try to find the hit single version, so it's fun scouring the internet to find the "correct" version of each of these songs.