Do MMO bosses count? If so, Absolute Virtue in FFXI. That thing remained undefeated for several years since no one could ever figure out the secrets required to weaken him. Also, after I had quit they shortly after released another ridiculous boss named Pandemonium Warden. I believe a Linkshell (basically a guild) went against it for 48 hours straight rotating players in and out and they ended up giving up.