Without a doubt, Absolute Virtue in FFXI.

To this day, Absolute Virtue remains undefeated on every single server. Not only that, but AV has very rarely been brought down to anything lower than 99% health.

The Kupo Linkshell on my server (Pandemonium) recently went head to head with Absolute Virtue, and with an army of 37 players they managed to get AV down to 98% health. They only managed that feat AFTER 3 1/2 HOURS OF BATTLING.

Currently, AV is referred to and nicknamed a bunch of things I can't post here at EoFF without getting in trouble. No one has a clue how to defeat AV. No one even knows if it's even possible to beat AV and maybe this is just a joke Squeenix is playing to see how much frustration they can squeeze out of the player base.

The GMs that are asked about Absolute Virtue usually respond by saying things like this.

It's my goal to be a part of the first team that manages to defeat Absolute Virtue. And when that's done, I'm quitting the damn game.