I wouldn't say this made me happiest, but I was fairly happy when I FC'd sana - Kotoba no Iranai Yakusoku. It's not really impressive at all, I was just happy since I thought I'd never FC because of streams and triplets.
my first try s on https://osu.ppy.sh/b/124264&m=1 after 1.5 years it is still my highest pp score and I still have no clue how I did it first try since i never fcd it again.
When I FC'd Okido Laboratory with acceptable acc and got an S rank. Really made me feel better about my aim, since I play with very small tablet area and was worried that my aim would suffer.