Hmm... I would say the time I beat Green Grass and High Tides on expert, 2 years after first trying it. I must have had a massive burst of luck, because not only did I gold star it without the score bonus from the 2nd solo, and not only did I only miss 4 notes in 'that bit' (if you've played it, you know what I mean), but I got so pumped up that I proceeded to beat Painkiller, Panic Attack and Visions right afterwards, none of which I had ever beaten before. My fingers started hurting halfway through Painkiller and just got worse, so I really have no idea how I did them all.
Had it actually happened, that would probably be surpassed by getting 100% in FFX. As in, getting every character's every stat to 255, getting the best equipment for every character (which I would have to make myself - around 150,000 Gil and between 5 and 20 hours per weapon/shield per character) and beating the Dark Aeons, every Monster Arena boss, Penance and Nemesis (to give you an idea of their toughness, characters with max stats will usually die in 1 attack to them, and even the most powerful form of the most powerful attack by the most powerful character at 255 everything would take 6 or 7 hits to kill them - and not only is that an Overdrive attack, it's based on slot reels, so luck is needed to use it once every few turns).
Unfortunately, I had filled in about half of the sphere grid (how you increase stats, for those who don't know) with +4 nodes (the best stat-increasing things available, and the only way to completely max everyone's stats) and beaten 2/8 Dark Aeons and about 80% of the Monster Arena bosses when my memory card conked out and decided it needed reformatting, and in doing so wiped all my save data.