Embarrassingly enough I think it was one of the 50-60 Samurai job quests. Probably the level 54 one. I remember just getting absolutely destroyed by the bosses Rage of Halone, and this was back when I first started playing. I had like 5k hp and the boss would do 3k every Rage of Halone. Third Eye wasn't cutting it to say the least. I didn't know what role actions were and I definitely wasn't geared properly. Surprisingly, that fight inspired me to really research the class to find out if I was doing something wrong, and that's when I realized just how important role actions and gear and rotations really were. It was a tough fight, and I had below 10k gil due to buying new gear, but gosh darnit I finally beat that guy! And boy was I proud!

I think that was one of the few times where I was really personally pushed on a job, and the habits that that quest inspired me to adopt really helped me out with learning Summoner and finally going into Savage.