Rayquaza in Sapphire. I blew the master ball on Latios or whichever one was in that game. Rayquaza is a pain in the butt due to his particular moveset:
Rest: Healed Status AND fully healed. Had to damage him again instead of tanking it out. Outrage: Does unholy amounts of damage, and confuses him which would cause Ray to knock himself out in the middle of the fight. ExtremeSpeed: Would out-priority my healing and stalling moves and kill the status inducer. Fly: Or he would just dodge the moves and fly up, forcing me to waste a turn until he hits the ground again so I can continue lobbing balls.
Level 70 with ridiculous offensive stats. When trying to catch him, all you can do is hope you get really lucky, since he'll more than likely kill himself.
Rest in Peace City of Heroes: April 27, 2004 - November 30, 2012. We will miss you. Cause of death: "Realignment of Company Focus" by NCsoft.