Before the concept of 'bonus bosses' began to proliferate, Final Fantasy offered some really tough Final Bosses.
Just getting to Zemus can be difficult, and once you're there, Zeromus is a real son of a ***** with that Big Bang attack. Exdeath was also a lot harder than I was expecting. His 'tree' form went down without all that much trouble, but Neo Exdeath brutalized me pretty badly, and two or three times I was just one more Almagest away from the Game Over screen.
By FFV, though, Square had begun to introduce bonus bosses like Omega, Shinryu, and the various WEAPON enemies that appear through the rest of the series, and even as early as the very next entry, FFVI, a decently-built party can usually cream the main dude with their eyes closed.
As a side rant, Exdeath doesn't get nearly enough credit as a villain. He slowly coalesced from the malice of thousands of sealed creatures into a single being of pure power. He levied such a powerful war machine that the combined military of the whole world wasn't enough to stop him, then enacted a trans-planetary cataclysm to rejoin two planets into one. He could possess people and spark huge natural disasters continents away just by willing it. Then he took over an entire dimension specifically used to house the most powerful threats to the universe, and uses that power to destroy the world- again! Even that wasn't enough for him, and the protagonists manage to stop him right before he fully manifests the power of the Void itself and subsumes the entire universe into nonexistence. Why? Just because he could.
And what do people say if you ask them about Exdeath? "He was some kind of tree-dude." *sigh*