Haha - I can't believe people are saying Omega from 8 is impossible without becoming invincible - he's really quite easy.
You just have to be quick (literally) and get some lionhearts/armageddon fists in and you're sorted. Dead easy, over in a few minutes.

Ruby and Emerald weapons are also fairly easy so long as you've prepared.

The master Pugs in 6 were probably the hardest enemies in 6 but they were still really easy when you could get something like 8 or 16 hits in or whatever it was.

Ozma was really very easy.

I'd say the only hard bosses are in FFX.

Even when you've maxed out and got all the ultimate weapons/armours Dark Bahamut and the 3 Dark Magus Sisters (all at once) are fairly tricky battles.

Then there's the hardest battle in any FF game - Penance.
The guy literally took me over an hour to beat. He's not so much hard (OK, he is pretty damn mean) as he is tedious. He takes ages and ages to kill and if you're not careful you'll lose the fight quite easily.

No contest.