I too think Vayne was a step in the right direction. Heck, I even lost to him once on lv 55, because he was so quick to use his special attacks. He was certainly more challenging than Kefka, Sephiroth and Ultimecia. I never beat X (got to BFA and then quit), but I remember Necron.
Oh yes, Necron is possibly the hardest Final Boss of the after-VII FFs. First time I faced him I lost 7 times in a row. Because of his damned Grand Cross and Neutron Ring. He's not superhard and with some preparation the battle can be pretty even, but there's no super quick way to beat Necron or Vayne.
Personally, I wish SE went back to the pre-VI days. Chaos, Cloud of Darkness, Zeromus and NeoExDeath were great challenges without being frustrating. It took preparation and some skill to beat them, not just the superuberabilities. (Though for Zeromus this changed pretty much in IV DS)
And I'm also not agreeing with how the superbosses are the greatest challenge. What is a superboss nowadays? A random beast with billions of HP and supermoves that all in all goes for a boring battle.
I beat Yiazmat and I swear I'll never do it again. It was such a damn pain in the behind because it was so BORING. Attack, revive, attack, revive. Save. Repeat.
Omega and Shinryuu. Those were great superbosses. They didn't have plenty of HP (less than the Final Boss) and fighting them required some strategy. You had to work with your jobs to find the good combination to fight them on even ground unless you wanted to be wiped out. I remember stealing all those Dragon Lances to beat Shinryuu (cheap, I know

) or working to get Spellblade so I could beat Omega. They were challenging, a bit cheap maybe, but much more fun than any superboss after VI.