The very specific one you're speaking of would be Yiazmat. I could not beat him without the use of a walkthrough, and that was a long time ago too (probably around 4 or 5 years back). The other notables were Zodiark, the final Esper to obtain, and Omega Mark XII with just over 10 mil. HP.

Okay, now if I could just see the differences in 1080i or 1080p please.