For me it was Zodiark, no doubt about it.
To start, there was the area: Henne Mines, Special Charter Dig (gosh, one of the rare locations I still remember the name). That damn area was full of bats with over 20 000 HP, my characters were easily killed. Then there were the hidden paths. I had to google a map or I'd be lost for hours.
Finally, there was the battle. Not only Zodiark was as strong (or stronger) than any other Esper, he had some amazing Darkja that could cause Instant Kill + Blind to all the party members. Oh, and it was special, since it wasn't absorbed by any Dark immune item. To make it extra nice, he was immune to every attack (except Scathe, if I'm not mistaken) for most of the battle.
...yeah, I think I'll stop now. Still epically awesome, though.
againstall has invadedBECKY = 10/10