I love superbosses too! They're typically the culmination of a good combat system in FF games. I beat a lot of them although there's still some unfinished business here and there.

My three favorite superbosses are all from FFX-2, go figure. Chac (non-Oversoul), Major Numerus and Strongest Shinra.

Chac is awesome because if you try to go at her with conventional strategies, you will DIE. Even if you know what you're in for - you will just DIE. There's not much to know about her anyway, her AI is a simple pattern. But her three attacks all require you to counter them specifically and not just overlook them. It was really fun trying to think of ways to beat her.

Major Numerus is HARD. I have little more to say about him. A boss who, on his second turn, uses an attack that ignores your defense and hits the entire party for 19000 damage? Hellish. Especially trying to fight him with YRP without Invincibility, or even without Mascot. It was quite a fun challenge though and I still can't believe I did it.

Shinra, finally, is my favorite. He's very hard (although Numerus is on a different level), intimidating, well-built-up and has a variety of attacks. His high defense means you have to think about how to tackle him rather than just brute-forcing him. His Auto-Regen means that you have to keep up the pressure at all times and not play for time. He has a simple pattern, but his attacks keep you on your toes and require you to strategize around them. Even then, if you go at him without Iron Dukes with just YRP, he's very tough. Plus, you can put him in your party after you beat him, which is smurfing awesome.

Those three are great. Sadly, the series has a lot of superbosses I don't care for at all, too. Penance, Trema and Yiazmat are all mainly a matter of patience and endurance rather than a quick but challenging test of how well you know to use the combat system's quirks to their full potential. Because that's what superbosses are to me - a challenge where you have to use your knowledge of the battle system to its fullest to overcome it.