1. Emerald serenades (358/2 days): I always hated chasing these things down.
2. Sheltering Zones (KH1): I could never hit get these guys to die without them splitting into three sea neons.
3. Large Bodies (multiple): Honestly though, any heartless that forces me to hit it's back to do damage, frustrates me.
4. Hot Rods (KH2): Ugh, I hated these. The first time I played through Timeless River, I must have died more than 5 times because of these things.
5. Ansem, Seeker of Darkness (KH1): He wasn't hard to beat but that Heartless Guardian of his made the fight incredibly annoying. Particularly when he would grab onto you and, if you hit the wrong button, paralyze you.
6. Mushroom XIII (KH2:FM+): Not sure if this counts as one or multiple (?) but these are really annoying.
7. Zexion Data (KH2:FM+): Again, not a necessarily difficult battle but having to go in and out of his lexicon drove me crazy.
8. Luxord (KH2): On a similar note, being turned into a die and a card was not fun either. It took me forever to defeat his data form since I'm horrible at timing the final quick time event.
9. Lingering Will (KH2:FM+): This was one of the hardest fights I've ever had the pleasure of playing. I beat him once and I won't be doing it ever again.
10. Trickmaster (KH1): I always disliked having to fight Trickmaster. Especially after playing KH2 with the superior battle system. It really makes you realize how outdated Kh1's battle system has become over the years, even though at the time it was one of the best.