5. Ugh-Zan III, Serious Sam: The First Encounter (PC, 2001)
One of the rules of first-person shooters developed in Eastern Europe seems to be "Include a stupidly big boss." While Painkiller's monstrous hey-where'd-the-horizon-go baddie almost muscled its way into the list, we reckon it was the first Serious Sam's end-boss that started the trend.
Above: If you look really closely, you might be able to make out a 300 foot-tall demon in the distance.
When the game's waves of cannon fodder suddenly stop battering themselves against you like exploding bugs across a windshield you know something big's coming - but you're not prepared for just how big Ugh-Zan III is. Technically he's only three hundred feet tall, but at the time you'll have little time for measuring.