Geez, after an entire week of looking through my memory bank for a first boss that actually gave me a rage quit, I have finally found one from my childhood that really ticked me off (hopefully you guys remember this action RPG).
The thirst quencher empire: Steam Guardian from Brave Fencer Musashi.
At first you Finnish your insane run from a giant bolder head all the way down from a 60-foot tower to a castle, Then all of a sudden after a dramatic scene with the princess being abducted, you face a freaken large steam powered knight that takes up half the room while he dropped pillars, chandeliers, fire rings, and that fairly accurate wrecking ball of his at you. At first I had no idea that you needed to attack his legs to have him fall for a few seconds to reveal his weak spot, then literally throw him to the next area when he shuts down after enough damage. After destroying all his four legs you need to carefully maneuver away from his leaps to reveal that small weak point of his that shows itself for about two seconds then a reapeation of the same leap/wrecking ball retaliation. Afterward everything is a cake walk once you corner him near a wall where he goes into his desperado leap exposing his spot more frequently(even though that spot limits your movement...again).
I have to say with enough practice, his pattern was pretty predictable, but hell... Square really knew how to warm you up in only the first 10 minutes of playing this game, and this was just a part of the beginning introduction to the game!