I have several superbosses that rank among my favorites from different games. Superbosses are always a fun thing for me to go after. I love their inclusions in games.
Final Fantasy V: Omega. This was hard because FFV has so many good superbosses like Shinryu, Neo Shinryu, and Omega MK II. But I always have the most fun with the original Omega. Especially setting up the Thundaga Rapid Fire technique.
Final Fantasy IX: Ozma. A very frustratingly hard super boss due to how random it is, but when you win it's so rewarding.
Final Fantasy VIII: Omega Weapon. I use to not like this boss, but now I kinda like it.
Kingdom Hearts: Phantom. Another boss I use to hate but now really love. Honorable mention to Sephiroth.
Kingdom Hearts II: It's likely one of the Data fights. Either Data Roxas, Data Marluxia, or Data Lexaeus. All are tough but fun to fight. Again honorable mention to Sephiroth.
Kingdom Hearts III: Data Xion. Super hard but not unfair, once you learn it it flows perfectly.
As for superbosses I hope to someday defeat.
Final Fantasy III: Iron Giant. Can't fight it without a second FFIII game. Want to fight it.
Final Fantasy XII: Yiazmat. Never made it to this thing.
Kingdom Hearts II: Lingering Will. I have fought him but never beat him, and lost interest when ReMind came out. Someday I'll return to it.
Kingdom Hearts 358/2 Days: Dustflier. I have attempted it but like Lingering Will I failed. Someday I'll do it.
Kingdom Hearts Birth By Sleep: Whatever the superboss is. I never looked for it due to the PSP hurting my hands. Maybe on the PS4.
Kingdom Hearts Dream Drop Distance: Same as BBS, but I haven't played it yet. When I do I'll hunt for superbosses.
Megaman Battle Network all games, all hidden bosses after the game: I'm not good enough to go beyond beating the main story. Hopefully someday I can to fight the superbosses.
Megaman Starforce, same as Battle Network.
Pokemon RSE, DPPt: Battle Frontier Brains. I dream of fighting these guys, but I'm not skilled to get through the numerous trainers to get to them due to the fixed 50 level limit so I have no unfair level advantage lol.