For me personal I had a lot of trouble with all of these bosses and this came down really close between Sephiroth from KH1 Vanitas's Lingering spirit from BBS (Birth by Sleep) and the Lingering Will from KH2 and what ultimately made the difference for me is Sephiroth gets easier if you can past his instant kill attack phase (I believe that is phase two) and in BBS you can equip moves that give you invincibility frames to make his fight easier. The hardest fight for me was the Lingering Will because you can only deal counter damage and you have to move fast to do that along with the fact that during his second phase he gives you almost no time to counter attack and his second phase move has enough of a space to between attacks to end second chance without you getting a chance to heal. 

Now I can't wait to hear about experiences with the secret bosses in the Kingdom Hearts series there are a lot of strong ones especially compared to the normal story bosses.

Personally I haven't got the chance to fight most of these, Since I don't have Final Remix anything. (Except Birth by Sleep)

But the most trouble I had was with Vanitas Remnant. I still haven't beat him because he just wrecks me in like 6 seconds... I don't even wanna try mysterious figure, even though I love the battle theme enough to try.