Leech Man from Resident Evil Outbreak: File #1 in "The Hive" scenario at Raccoon City Hospital. None of your weapons can even scratch him, all you can do is temporarily distract him with a blood pack (which are in limited supply), and he hounds you through the entire level by traveling through the duct work (now that's what I call real survival horror). Eventually, you can lure him into a special high temperature room in the lowest level of the hospital's basement and roast him, but, until then, all you can do is run for your life. Worse, if any of your teammates are killed during the scenario, they also become Leech Men/Women, and unlike the original, there is absolutely no way to get rid of them, as they aren't dumb enough to fall for the same trick as the first one. Leech Man killed me constantly and ticked me off so much that I literally did not play the game again for years. I've beaten him many times now (the recent Capcomical contest gave me the nudge I needed to try the game again, FAQ in hand), but when I first got Resident Evil Outbreak, he was an absolute killjoy gamebreaker for me.